On November 29 2000, a majority of the United States Court of Appeals for the Federal Circuit, sitting en banc, rendered an opinion in Festo Corp v Shoketsu Kogyo Kabushiki Co, published at 56 USPQ 2d 1865, which effectively extinguishes the application of the doctrine of equivalents to any term of a patent claim that was narrowed by amendment during its prosecution before the US Patent and Trademark Office (USPTO). While the majority opinion does purportedly limit the prosecution estoppel created by amendments that narrow a claim in any respect to those amendments made for reasons "related to patentability" , that opinion is also unequivocally clear in holding that any narrowing amendment to a claim term made for "any reason affecting the issuance of a patent" (56 USPQ 2d at 1870-1871) is "related to patentability" whether made voluntarily or in response to a rejection. Specific mention is made of amendments made for reasons based on 35 USC § § 101 and 112, as well as for prior art reasons based on 35 USC § § 102 and 103 as giving rise to prosecution estoppels and thus foreclosing application of the doctrine of equivalents to amended terms or expressions in claims. The majority opinion leaves an apparent escape hatch from estoppel for narrowing claim amendments made for reasons other than patentability, provided each such reason is stated in the prosecution record at the time of the narrowing amendment but this is of very little practical effect because reasons for narrowing amendments to claims that do not somehow implicate patentability are extremely hard to conceive of, much less substantiate. The majority opinion is explicit in holding that: "When a claim amendment creates prosecution history estoppel with regard to a claim element, there is no range of equivalents available for the amended claim element. Application of the doctrine of equivalents to the claim element is completely barred" (56USPQ2d at 1872).
