Cite as: 535 U. S. 722 (2002)
Opinion of the Court
established that the inventor turned his attention to the subject matter in question, knew the words for both the broader and narrower claim, and affirmatively chose the latter.
A
The first question in this case concerns the kinds of amendments that may give rise to estoppel. Petitioner argues that estoppel should arise when amendments are intended to narrow the subject matter of the patented invention, for instance, amendments to avoid prior art, but not when the amendments are made to comply with requirements concerning the form of the patent application. In Warner-Jenkinson we recognized that prosecution history estoppel does not arise in every instance when a patent application is amended. Our "prior cases have consistently applied prosecution history estoppel only where claims have been amended for a limited set of reasons," such as "to avoid the prior art, or otherwise to address a specific concern—such as obviousness—that arguably would have rendered the claimed subject matter unpatentable." 520 U. S., at 30-32. While we made clear that estoppel applies to amendments made for a "substantial reason related to patentability," id., at 33, we did not purport to define that term or to catalog every reason that might raise an estoppel. Indeed, we stated that even if the amendment's purpose were unrelated to patentability, the court might consider whether it was the kind of reason that nonetheless might require resort to the estoppel doctrine. Id., at 40-41.
Petitioner is correct that estoppel has been discussed most often in the context of amendments made to avoid the prior art. See Exhibit Supply Co., supra, at 137; Keystone Driller Co. v. Northwest Engineering Corp., 294 U. S. 42, 48 (1935). Amendment to accommodate prior art was the emphasis, too, of our decision in Warner-Jenkinson, supra, at 30. It does not follow, however, that amendments for other purposes will not give rise to estoppel. Prosecution
735
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