In analyzing whether the "tangential relationship" exception to the doctrine of equivalents estoppel applied, the U.S. Court of Appeals for the Federal Circuit affirmed-in-part, reversed-in-part and remanded a district court's summary judgment that the defendant did not infringe the patents directed to methods for detecting chromosomal abnormalities under the doctrine of equivalents. Regents of Univ. of Cal. v. DakoCytomation Cal., Inc., Case No. 07-1202 (Fed. Cir., Feb. 28, 2008) (Lourie, J.; Prost, J., dissenting-in-part).
The Regents of the University of California and Abbott Molecular Inc. and Abbott Laboratories Inc., as the owner and exclusive licensees of the patents-in-suit, brought an infringement action and filed a motion for preliminary injunction seeking to enjoin the manufacture and sale of several accused products. The patents in dispute claimed methods of blocking and removing repetitive nucleic acid fragments so to allow highly specific measurement of chromosomal abnormalities originating from within a cell. The district court denied the preliminary injunction, finding appellants unlikely to succeed on the merits and granted summary judgment, in part, on the construction of the claim limitation "blocking nucleic acid"—a term required by all the claims of one of the patents-in-suit. The parties stipulated that "blocking nucleic acid" means "fragments of repetitive-sequence-enriched DNA or RNA." Because the accused kits do not use human DNA and instead use synthetic nucleic acids referred to as peptide nucleic acids (PNA), the district court concluded that those products do not literally infringe. The lower court then considered whether those products infringe under the doctrine of equivalents. After determining that the patentees narrowed the scope of the "blocking nucleic acid" limitation during prosecution, the court concluded that appellants were barred from asserting that PNAs were an equivalent of a "blocking nucleic acid" and granted summary judgment of non-infringement under the doctrine of equivalents. Patentee appealed.
On appeal, the Federal Circuit noted that "[t]he inquiry into whether a patentee can rebut the Festo presumption under the 'tangential' criterion focuses on the patentee's objectively apparent reason for the narrowing amendment." With regard to the claim limitation "blocking nucleic acid," the Court held that, because the accused products utilized peptide nucleic acids instead of the claimed DNA/RNA fragments, there was no literal infringement. However, the Court vacated summary judgment and held in error the lower court's finding that prosecution history estoppel barred appellants from asserting infringement under the doctrine of equivalents. Dako argued that during prosecution the appellants had narrowed the corresponding application to include only "blocking nucleic acids" and accordingly, had surrendered the peptide nucleic acid equivalent of the accused products. The Court noted that the patentees identified two main reasons for the narrowing amendment: to "facilitate prosecution" and, more significantly, to distinguish the invention over the prior art. According to the Court, the prosecution history revealed that in narrowing the claim to overcome the prior art rejections, the focus of the patentees' arguments centered on the method of blocking—not on the particular type of nucleic acid that could be used for blocking. The facts that the "nucleic acid" limitation was never at issue during prosecution and that that none of the cited references concerned the type of nucleic acid that could perform the blocking was sufficient for the Court to conclude that the lower court erred in concluding that the appellants are precluded by estoppel from asserting that Dako's products infringe under the doctrine of equivalents.
Judge Prost dissented with respect to the holding that tangential exception to prosecution history estoppel applies and argued that the appellants could reasonably have been expected to have drafted a claim that encompassed blocking repetitive sequences using PNA.
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