FDA Advances A "Plausible Mechanism" Framework For Rare Disease Drug Development And Signals A Shift To A Single Trial With Confirmatory Evidence Default For All Drugs

On February 23, 2026, the U.S. Food and Drug Administration (FDA) reaffirmed its commitment to facilitating the development of therapeutics for rare diseases by issuing a draft guidance on a "plausible mechanism framework" for individualized therapies. This draft guidance followed on the heels of an opinion piece by FDA Commissioner Makary and CBER Director Prasad that was published last week in the New England Journal of Medicine (NEJM), in which the authors signaled that FDA's default position going forward — even for treatments for common diseases — will be one adequate and well-controlled study with confirmatory evidence. We discuss both of these developments below.

Plausible Mechanism Framework for Individualized Therapies

FDA's new draft guidance, "Considerations for the use of the Plausible Mechanism Framework to Develop Individualized Therapies that Target Specific Genetic Conditions with Known Biological Cause" (the Plausible Mechanism Framework Draft Guidance), introduces a framework intended to help facilitate the development and approval of "individualized therapies," drugs and biological products that target a specific pathophysiologic abnormality serving as the root cause of a disease, and may include genome editing and certain RNA-based therapies.¹ Historically, developing these treatments has been challenging in part because randomized controlled trials often are not feasible due to very small patient populations (i.e., patients with rare and ultra-rare diseases).² The Plausible Mechanism Framework Draft Guidance lays out FDA's proposed thinking on a streamlined evidence generation framework for establishing substantial evidence of efficacy to support approval of these drugs. It also provides FDA's recommendations for clinical and non-clinical studies intended to demonstrate the safety and effectiveness of these kinds of individualized therapies, and potentially others, as well.

The Plausible Mechanism Framework Draft Guidance was announced by Commissioner Makary in the opening remarks at FDA's Rare Disease Day,³ in an FDA press release,⁴ at a U.S. Department of Health and Human Services (HHS) press conference,⁵ and in an FDA Direct roundtable on X.⁶ It follows a 2025 NEJM article on the same subject co-authored by Commissioner Makary and FDA's Center for Biologics Evaluation and Research (CBER) Director and FDA Chief Medical and Scientific Officer Vinay Prasad (see Arnold & Porter's previous Advisory). Though originally termed a "pathway" by Makary and Prasad,⁷ the Plausible Mechanism Framework Draft Guidance is clear that FDA is not crafting a new pathway or a new standard of review. Rather, the draft guidance establishes a framework for meeting the existing statutory standard of "substantial evidence of efficacy" and evidence of safety in a way that facilitates a path to approval for this narrow group of therapies for which there are too few patients to enroll enough participants in a clinical trial, or where first-in-human studies may also serve as pivotal trials to support approval.⁸

The Plausible Mechanism Framework Draft Guidance prioritizes genome editing therapies (GEs) and RNA-based therapies (e.g., antisense oligonucleotides (ASOs) and small interfering RNAs) that are intended to treat rare, severely debilitating, or life-threatening diseases (SDLTs)) — ostensibly a narrower category than the "serious or life-threatening" diseases and conditions that define the scope of many of FDA's expedited programs.⁹ And the Plausible Mechanism Framework Draft Guidance concentrates on the development of these kinds of therapies for diseases that have a well-characterized, identifiable molecular or cellular abnormality, and therapies that target the underlying abnormality, its proximal pathogenic pathway, or a well-characterized downstream or compensatory mechanism with a clear mechanistic rationale.¹⁰ However, the draft guidance does not limit the plausible mechanism framework just to GEs or RNA-based therapies.¹¹ The Plausible Mechanism Framework Draft Guidance itself, the accompanying FDA press release, and CDER Director Høeg's comments during the HHS Press Conference all acknowledge that other kinds of therapies, including therapies for common diseases, could also leverage the plausible mechanism framework.¹²

The Plausible Mechanism Framework. The Plausible Mechanism Framework Draft Guidance describes a framework centered around five key elements which, if met, FDA believes will indicate that the therapy is "plausibly" effective to support approval.

1. Identification of a specific genetic, cellular, or molecular abnormality with a clear connection between specific alteration and disease indication. The draft guidance focuses on therapies for diseases or conditions that are caused by identifiable genetic variants for which GEs or RNA-based therapies can pinpoint the abnormality, its proximal pathogenic pathway, or some other causative mechanism "with a clear mechanistic rationale."¹³ In practice, this means that the utility of the plausible mechanism framework will be limited to diseases or conditions with a known cause, and to therapies that target that specific cause. The Agency specifically notes that the sponsor's submission must "adequately demonstrate that the improvement in outcome cannot reasonably be attributed to alternative treatments or natural variability in the disease phenotype."¹⁴
2. Development of a therapy that targets the underlying or proximate pathogenic biological alterations. The draft guidance is focused on therapies that address the "how" of a disease or condition, for example, gene editing technologies that are designed to correct gene mutations and can be modified based on what targets are necessary to address.¹⁵ Thus, if a sponsor develops a product that uses CRISPR technology as an example, the plausible mechanism framework suggests that the sponsor can submit a single Investigational New Drug Application (IND)/Biologics License Application (BLA) covering multiple targets within a single gene "if the method of correction is the same between the gRNA (e.g., return to the normal/native gene sequence)."¹⁶
3. Reliance on a well-characterized natural history of the disease in an untreated population. The draft guidance leans heavily on well-characterized natural histories being available to inform clinical study design and as external controls. This element is crucial to FDA's determination of effectiveness. FDA must be able to determine that the patient's improvement was due to the intervention rather than a natural change in the disease or condition. As such, there are many rare diseases or conditions that will be beyond the scope of this new framework at least until the natural history of the disease or condition becomes more established.
4. Confirmation that the target was successfully drugged, edited, or both. FDA will also look to the sponsor to demonstrate that the therapy had the desired effect on the patient. As mentioned, the Plausible Mechanism Framework Draft Guidance suggests that this element will require a strong understanding of the natural course of the disease. FDA also envisions in the draft guidance that confirmatory evidence may include: mechanistic or pharmacodynamic data; confirmation of target engagement based on nonclinical or clinical data; and/or exposure-response on biomarkers and clinical outcomes.¹⁷
5. Demonstration of improvement in clinical outcomes or course. Showing FDA that this last key element is achieved hinges largely on the ability of the sponsor to define endpoints connected to a specific clinical outcome or change in disease course. To this end, FDA recommends that sponsors begin observational protocols as early as possible with potential study participants to allow for pilot assessments, including establishing the lead-in baseline and to facilitate identification of disease-relevant biomarkers.¹⁸

Regulatory Framework for Approval and Post-Marketing Requirements. The Plausible Mechanism Framework Draft Guidance maintains that, as with all drug applications, FDA approval will ultimately depend on the benefit-risk assessment of the application. How an application meets the statutory standard of safety and substantial evidence of effectiveness (and generates the specific types of evidence required) will depend on the investigational product and its intended use(s).¹⁹ However, the Plausible Mechanism Framework Draft Guidance explains that safety will be considered "in the context of the underlying SDLT," and that substantial evidence of effectiveness will be established through one adequate and well-controlled clinical investigation with confirmatory evidence.²⁰ And the draft guidance explains that, when determining whether a new individualized therapy meets the statutory standard (substantial evidence of effectiveness), the Agency will consider the clinical context of the disease and the level of unmet need, and will assess how challenging enrollment is for a clinical trial based on these facts.²¹

Individualized therapies that are approved under the draft guidance's framework may be approved through the traditional approval pathway²² or accelerated approval pathway, but in either case, FDA is expected to impose significant postmarketing commitments on sponsors of such products — both to supplement small safety data sets and to "include collection of efficacy outcomes if there is evidence of a potential for loss of efficacy over time."²³ For instance, considering that there will be limited safety data available at the time a marketing application is submitted, the draft guidance anticipates that FDA will likely require sponsors to collect post-market safety data — though details on how this will work are not provided and the draft guidance cites only to the accelerated approval authorities for this proposition.²⁴

In a nod to the fact that many GEs may also be platform technologies,²⁵ the Plausible Mechanism Framework Draft Guidance notes that multiple product "variants" may be included in a single IND or BLA — and that once a product is licensed, additional GE product variants related to other mutations can be "add[ed]" based on a "highly supported 'plausible' mechanism of action."²⁶ We also note that the draft guidance suggests places in which FDA may leverage data or information across product applications. For instance, the draft guidance notes that nonclinical information outside of a new drug application may be used to "inform safety" of an ASO.²⁷ And it provides that confirmatory evidence may leverage data from other clinical programs using the same or similar technologies (e.g., a program with a different genome editor version or multiple gRNAs).²⁸

Designing Development Programs under the Plausible Mechanism Framework. "Substantial evidence" of efficacy historically has required two adequate and well-controlled investigations that are blinded, randomized, and placebo-controlled, and enable a direct assessment of clinical benefit,²⁹ though FDA has long used flexibility to approve drugs intended to treat rare diseases based on a single trial with confirmatory evidence. (For a discussion of the Agency's new default policy on one adequate and well-controlled investigation with confirmatory evidence, see Part II of this Advisory). The Plausible Mechanism Framework Draft Guidance maintains that relevant study protocols should meet regulatory requirements for adequate and well-controlled studies (e.g., 21 C.F.R. Parts 314, 50, and 56). It also provides a number of recommendations related to the design and conduct of clinical studies, such as:

• Prior to treatment initiation, diagnoses should be appropriately confirmed with validated testing, and there should be evidence that the targeted genetic variant(s) play a causal role in the disease.
• Sponsors should carefully select doses, including through use of pharmacodynamic biomarkers as appropriate, or other available methodologies. Trials should be designed to collect outcomes data that demonstrate clinical benefit, including outcomes that can be validated by natural history studies.
• Biomarkers should be selected to support proof of concept, confirm target engagement, inform the need for dose escalation, and identify safety issues. They should be assessed with respect to direct measures of target engagement or primary pharmacodynamics—which FDA notes is "likely essential."
• Safety monitoring should be informed by nonclinical findings and previously identified risks associated with "similar products."³⁰

Ultimately, results should be "robust" enough to exclude false signals of effectiveness that may appear among a small sample size.³¹ The Plausible Mechanism Framework Draft Guidance also maintains flexibility in designing clinical development programs for these treatments, including anticipating that first-in-human studies may also serve as pivotal trials to support approval, that trials can use a disease's natural history data as an external control, allowing for use of "master protocols (e.g., umbrella or platform trials)" for studies concerning therapies for different genetic changes within the same disease.³² Commissioner Makary also suggests that there is room for the use of Real World Evidence in the plausible mechanism framework (and more generally across drug development).³³

The Plausible Mechanism Framework Draft Guidance reflects similar flexibility for non-clinical data generation. It recommends that the nonclinical studies demonstrate the feasibility of the product's proposed route of administration, support the scientific rationale for the product's administration, and identify potential product risks, for instance.³⁴ It notes that FDA may use its discretion toward nonclinical study design as well, such as exercising flexibility toward study duration, types of studies needed, and model selection, and permit sponsors to leverage data and knowledge between clinical programs using multiple gRNAs and/or editor combinations.³⁵

Moreover, the Plausible Mechanism Framework Draft Guidance emphasizes that chemistry, manufacturing, and controls (CMC) development will be key to approval under the plausible mechanism framework given the shorter clinical investigation phase expected for individualized therapies. The draft guidance recommends that sponsors develop CMC alongside clinical development, consider various aspects of the CMC process (e.g., scale, validation, commercial feasibility) to support the initial IND, validate the manufacturing process to support a marketing application for a drug or biologic, and ensure that all assays used for release and stability testing comply with applicable marketing application and current Good Manufacturing Practice regulations.³⁶

Early Engagement with FDA. FDA recommends that sponsors interested in submitting an application based on the recommendations of the draft guidance discuss their plans with the relevant review division, including their nonclinical and CMC plans.³⁷ The Agency's press release notes that FDA intends to prioritize regulatory flexibility to help get new treatments to patients as quickly as possible. Thus, there may be opportunities for case-by-case enforcement discretion (or other types of regulatory flexibility) that come from early conversations with FDA about methodologies for bringing innovative new therapies to market based on these new recommendations.

Request for Comments. FDA invites stakeholders to submit comments to the draft guidance by April 27, 2026.³⁸ Based on the foregoing, industry could consider submitting comments to inquire about how to design a first-in-human study as the adequate and well-controlled clinical investigation to support a marketing application, how FDA purports to examine a marketing application that includes data from a collection of diseases and/or treatments within a single trial, FDA's authority to approve drugs based on postmarket data, and how FDA will enforce postmarketing requirements and commitments. Some sponsors may be interested in how FDA's draft guidance affects their pending development programs or applications, while others that recently received Complete Response Letters might request clarity or flexibility with respect to their resubmissions.

Single Trial with Confirmatory Evidence Default Standard for All Drug Development Programs

In a February 18, 2026, NEJM article, Makary and Prasad announced a(nother) major policy shift impacting the implementation of the substantial evidence of efficacy requirement for drug approvals.³⁹ Although the article is not formal Agency guidance, the authors proclaim, "[g]oing forward, the FDA's default position is that one adequate and well-controlled study, combined with confirmatory evidence, will serve as the basis of marketing authorization" for new drugs.⁴⁰ Makary initially previewed FDA's plans to shift the default requirement to a single clinical study with confirmatory evidence in December 2025.⁴¹

FDA has had the authority to approve new drugs based on a single trial with confirmatory evidence for nearly 30 years, and it has exercised that power expansively in arenas like rare disease and oncology. By making that standard the Agency's default policy, however, Makary and Prasad are opening the "single trial" standard to drugs intended to treat common diseases and conditions as well. And, crucially, they are shifting the burden onto FDA review teams to justify multiple trials whereas, until now, sponsors have had to demonstrate to FDA that they could meet the substantial evidence standard with a single trial and confirmatory evidence.

Origins of the Two-Trial Standard and Single-Trial Exception

"Substantial evidence," as defined in the FD&C Act, means "evidence consisting of adequate and well-controlled investigations, including clinical investigations ... on the basis of which it could fairly and reasonably be concluded ... that the drug will have the effect it purports or is represented to have under the conditions of use prescribed, recommended or suggested in the labeling or proposed labeling thereof."⁴² Through regulations and guidance, FDA has interpreted the substantial evidence requirement to generally necessitate more than one adequate and well-controlled investigation, each "convincing on its own," to establish effectiveness. Multiple trials can help assuage concerns about reproducibility that can arise from a single trial, as well as potential impacts from external influences (e.g., bias and chance).⁴³

The Food and Drug Administration Modernization Act of 1997 (FDAMA), gave FDA the authority to approve a drug based on one adequate and well-controlled investigation with confirmatory evidence. It added to section 505(d) of the FD&C Act that FDA, at its discretion, could "make exception to the general requirement that there must be more than one adequate and well-controlled investigation to support an effectiveness determination"⁴⁴ to accept data from "one adequate and well-controlled clinical investigation and confirmatory evidence" to establish substantial evidence of efficacy.⁴⁵ FDA has accordingly relied on that authority to approve drugs for oncology and rare diseases, for example. Indeed, roughly half of the drugs approved last year were supported by one Phase 3 study. FDA has also issued draft guidance to describe when a single adequate and well-controlled clinical investigation and confirmatory evidence may be appropriate to demonstrate substantial evidence of effectiveness (though that draft guidance reiterates that this exception does not supplant FDA's default view that the substantial evidence requirement generally requires two adequate and well-controlled clinical investigations).⁴⁶

FDA's New Default Policy

Makary and Prasad declare in their opinion piece that the Agency's policy going forward should be to accept the exception as the norm. FDA's historical reliance on the two-adequate-and-well-controlled-investigation standard is inappropriate in today's "modern world," they assert, where "drug discovery becomes increasingly precise and scientific." Rather, "in 2026, there are powerful alternative ways to feel assured that our products help people live longer or better than requiring manufacturers to test them yet again," particularly "[w]hen a drug is a game-changer, you can see the effect from space."⁴⁷ Thus, Makary and Prasad predict that an Agency shift to a default of one adequate and well-controlled clinical investigation with confirmatory evidence will "substantially reduce costs for sponsors," "speed drugs to market," and potentially lower drug prices.⁴⁸

Importantly, the authors go to lengths to assure that such a new policy would still maintain FDA's drug approval standard. They emphasize that approval decisions will turn on the quality of the single "robust and sound" study. FDA will "carefully examine all aspects of study design with particular focus on controls, end points, effect size, and statistical protocols."⁴⁹ They encourage sponsors to ensure that the single study is "credibl[e]," emphasizing considerations such as "the magnitude of the effect, the use of a contemporary control group (versus a historical one), the nature of the control group (is it the best available therapy?), the prespecification of a hypothesis, the choice of a primary end point, the concordance with biologic correlates (including evidence of alteration of an in vivo target), alignment of intermediate end points, statistical power, blinding, concealment, independent review, whether post-protocol therapy is on par with the U.S. standard of care, the use of concomitant therapy, inclusion criteria, exclusion criteria, randomization, run-in periods, how missing data are handled, and many additional factors."⁵⁰ FDA may require two adequate and well-controlled clinical studies, they explain, if data from one trial is insufficient (e.g., if an intervention "has a nebulous, pluripotent, or nonspecific mechanism of action" or "affects a labile, short-term, or surrogate outcome," or if a trial "has some underlying limitation or deficiency").⁵¹

Considerations for the New Policy Going Forward

Once implemented, whether the impact of the new default policy will be meaningful will largely turn on whether FDA review divisions believe additional products can be approved via a single study and confirmatory evidence available at the time of approval to resolve residual questions and manage risk. While that is increasingly the case with sophisticated new modeling and other tools, the speed at which this will actually become the predominant approach is uncertain. Companies may need to be careful about relying on assumptions relating to the adequacy of a one study clinical development plan.

Should the "single trial" policy follow the path paved by the "plausible mechanism" one, with a NEJM opinion piece followed by Agency guidance attempting to effectuate the policy, we may see guidance on this topic soon. There are many topics not addressed by the NEJM article that FDA will have to grapple with if it is to formalize a single trial policy — including that the two-trial policy is enshrined in Agency guidance and reflected in FDA's regulations. We will be watching to see whether, and the extent to which, FDA takes on the following topics, and likely others as well.

• Confirmatory Evidence. The NEJM article does not address how FDA will handle the statutorily required confirmatory evidence. This will be critical if the revised policy is to take hold. Makary and Prasad state only that confirmatory evidence "can include mechanistic science, data from a related indication, animal models, information from other drugs of the same class, real-world evidence, or a second adequate and well-controlled study."⁵² Turning to prior Agency guidance to offer additional clues, FDA may look carefully at the quality and quantity of the confirmatory evidence, including whether the confirmatory evidence is appropriate to support the specific development program.⁵³
• Postmarketing Authorities. Makary and Prasad seem to rely heavily on the Agency's ability to impose post-market confirmatory studies if and when the Agency thinks they are warranted. But FDA's authorities are actually quite limited in this way. For example, FDA cannot require additional studies after a drug is approved unless specific criteria are met.⁵⁴
• Timing and Adherence to Prior Advice. If this new "default" is to become Agency policy, it will be critical for FDA to articulate a time frame for when the new standard will become effective. Drug developers are relying on FDA's two-trial standard now to design and pursue development programs, and a new default standard has the potential to seriously affect competitive scenarios in which "first" approvals are meaningful (for example with respect to exclusivities or priority review vouchers). It will be critical to ensure that sponsors understand the timing of this new policy and the extent to which it supersedes advice given by FDA to date.
• The Status of FDA Regulations and Guidance. Will FDA withdraw or modify existing regulations and guidance that reflect an interpretation of the substantial evidence of efficacy requirement as demanding two clinical investigations in most cases?
• The Resource Burden. This article comes amidst ongoing user fee negotiations at FDA. Meanwhile, it is far from clear that FDA has adequate resources to implement the new default standard, considering that this policy shifts the burden of justifying a single trial and confirmatory evidence from the drug developer to the FDA. Will user fees be adjusted accordingly?
• Impacts on Reimbursement. With a smaller quantum of evidence for FDA approval, sponsors may still need to generate additional data for reimbursement. This may be outside of FDA's purview, but it will be a critical piece of the puzzle if investment is truly to be spurred.

While guidance on this potential new policy has not yet been published, we believe it will be essential for sponsors to engage early with the Agency.⁵⁵

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Comments on the Plausible Mechanism Framework Draft Guidance are due on April 27, 2026.

Footnotes

1. U.S. Food & Drug Admin., Considerations for the use of the Plausible Mechanism Framework to Development Individualized Therapies that Target Specific Genetic Conditions with Known Biological Cause – Draft Guidance for Industry (Feb. 2026).
2. Id. at 3.
3. FDA's Rare Disease Day 2026 – An Event for Patients, YOUTUBE (U.S. Food & Drug Admin.) (Feb. 18, 2026).
4. U.S. Food & Drug Admin., Press Release, FDA Launches Framework for Accelerating Development of Individualized Therapies for Ultra-Rare Diseases (Feb. 23, 2026).
5. Advancing Innovation on Rare Disease Therapies, YOUTUBE (U.S. Dep't of Health & Human Servs.) (Feb. 23, 2026).
6. Marty Makary (@DrMakaryFDA), X (23, 2026 11:45 AM).
7. See Vinay Prasad & Martin A. Makary, FDA's New Plausible Mechanism Pathway, 393 N. ENG. J. MED. 2365 (Nov. 12, 2025).
8. See Plausible Mechanism Framework Draft Guidance, at 1 and 3.
9. Id. at 3-4.
10. Id. at 4.
11. Id. at 2.
12. Id.; FDA Launches Framework for Accelerating Development of Individualized Therapies for Ultra-Rare Diseases; Advancing Innovation on Rare Disease Therapies (58:40-59:18).
13. Plausible Mechanism Framework Draft Guidance, at 4.
14. Id.
15. Id. at 5.
16. Id.
17. Id. at 6.
18. Id. at 12.
19. Id. at 3-4.
20. Id. at 4.
21. Id. at 5.
22. Id. at 6 n.17.
23. Id. at 6.
24. Id.
25. See U.S. Food & Drug Admin., Platform Technology Designation Program for Drug Development Guidance for Industry 11 (May 2024) (draft).
26. Plausible Mechanism Framework Draft Guidance, at 5.
27. Id. at 10-11.
28. Id. at 9.
29. 21 U.S.C. § 355(d); 21 C.F.R. § 314.126(a)-(b).
30. Plausible Mechanism Framework Draft Guidance, at 14-17.
31. Id. at 6 and 12-17.
32. Id. at 12-17.
33. Id. at 14; Advancing Innovation on Rare Disease Therapies (1:00:25-1:01:27).
34. Plausible Mechanism Framework Draft Guidance, at 7.
35. Id. at 9.
36. Id. at 17-20.
37. Id. at 2, 8, and 20.
38. 91 Fed. Reg. 9283, 9283 (Feb. 25, 2026).
39. Vinay Prasad & Martin A. Makary, One Pivotal Trial, the New Default Option for FDA Approval – Ending the Two-Trial Dogma, 394 N. ENGL. J. MED. 815 (Feb. 2026).
40. Id. at 816.
41. See Lizzy Lawrence, FDA to lower number of trials required for approval of drugs, other medical products, STAT (Dec. 4, 2025).
42. 21 U.S.C. § 355(d)
43. See 21 C.F.R. § 314.126; U.S. Food & Drug Admin., Demonstrating Substantial Evidence of Effectiveness With One Adequate and Well-Controlled Clinical Investigation and Confirmatory Evidence – Guidance for Industry 2 (Sept. 2023) (draft); U.S. Food & Drug Admin., Demonstrating Substantial Evidence of Effectiveness for Human Drug and Biological Products – Guidance for Industry 4 (Dec. 2019) (draft); U.S. Food & Drug Admin., Guidance for Industry – Providing Clinical Evidence of Effectiveness for Human Drugs and Biological Products 3 (May 1998).
44. 63 Fed. Reg. 27093, 27094 (May 15, 1998).
45. 21 U.S.C. § 355(d).
46. Demonstrating Substantial Evidence of Effectiveness With One Adequate and Well-Controlled Clinical Investigation and Confirmatory Evidence, at 2.
47. One Pivotal Trial, the New Default Option for FDA Approval, at 815-16.
48. Id. at 816.
49. Id.
50. Id. at 815.
51. Id. at 816.
52. Id. at 815.
53. See Demonstrating Substantial Evidence of Effectiveness With One Adequate and Well-Controlled Clinical Investigation and Confirmatory Evidence, at 4.
54. See, e.g., 21 U.S.C. §§ 355(o)(3) and 356(c).
55. See Demonstrating Substantial Evidence of Effectiveness With One Adequate and Well-Controlled Clinical Investigation and Confirmatory Evidence, at 3.

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