Last month, the Federal Circuit heard an appeal from an inter partes review proceeding concerning the validity of U.S. Patent No. 7,340,597 (the '597 Patent), which describes network security technology. The named inventor of the '597 Patent is Dr. David Cheriton, who was an employee of Cisco Systems, Inc. (Cisco) at the time of invention, and assigned his rights in the patent to Cisco. He later left Cisco to found Arista Networks, Inc. (Arista), and it was his new company, Arista, that challenged the validity of the patent he had assigned to Cisco.

Cisco argued that the Patent Trial and Appeal Board should not have permitted Arista to challenge the validity of the '597 patent under the common-law doctrine of "assignor estoppel," which holds that a party who assigns a patent to another, as well as his or her privies (such as the inventor's new employer), may not later challenge the validity of the assigned patent. Conversely, Arista argued that the statutory language governing inter partes review precluded application of the assignor estoppel doctrine.

The Federal Circuit agreed with Arista, relying on 35 U.S.C. § 311. Section 311(a) provides that "a person who is not the owner of a patent may file . . . a petition to institute an inter partes review." The Federal Circuit explained that the "plain language of this statutory provision is unambiguous" and precludes application of assignor estoppel in the inter partes review context. The court further reasoned that, in related contexts, Congress has expressly preserved equitable common law doctrines and did not do so here.

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