In response to a request by either a patentee or another interested party, the USPTO will reexamine an issued patent in light of prior art that creates a “substantial new question of patentability” compared to issues considered during the original examination. Accused infringers in patent litigation are increasingly requesting reexaminations of asserted patents, recognizing reexamination as a relatively low-cost, potentially high-impact adjunct to a lawsuit. A similar procedure, “reissue,” allows a patentee to apply for a reissued patent based upon an error made during the original prosecution, including the patentee claiming more or less that he or she had a right to claim.

Reexamination may result in changes to the claims, but may not broaden claim scope. By contrast, a reissue allows the patentee to not only narrow claims (as in reexamination), but to broaden claims as well if the reissue application is filed within two years of the issue date of the original patent. Under either post-issuance procedure, a claim amendment [1] may impose an undue hardship on an infringer of the reexamined or reissued claim where the infringer relied upon the scope of the original claim in attempting to avoid infringement of the original patent. To account for this hardship, “absolute” and “equitable” intervening rights are available to protect infringers from liability where claims are substantially changed during reexamination or reissue.[2]

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