What’s Estoppin’ You? – Beware the Barriers of Asserting Judicial Estoppel

Judicial estoppel is an equitable doctrine that precludes a party from taking a position in one proceeding that is inconsistent with a position the party took in a previous proceeding. It can be used as a defensive strategy in an inter partes review, for example, when the patent owner identifies that the petitioner is advancing an argument in its petition that is inconsistent with a position the petitioner took in some other proceeding. However, four recent IPRs, exemplified by IPR2018-00291, highlight that meeting the standard for judicial estoppel is no easy task. 

The technology at issue in the -00291 and related IPRs had to do with a sequencing-by-synthesis technique. The patent owner had four granted patents, each directed to a nucleotide analogue designed with a chemically cleavable linker on its sugar group and a unique identifier. A petitioner filed four petitions for IPR of the four patents and asserted that the use of the claimed linker would have been obvious. Briefly, the petitioner took the position that the asserted references disclosed the use of an allyl removable blocking group on a nucleotide’s sugar group and that this blocking group fell within the scope of the “chemically cleavable linker” in the claims at issue. In an attempt to defeat the petition, the patent owner asserted that the petitioner was estopped from making this obviousness argument because it contradicted statements the petitioner made in a previous ex parte reexamination.

The patent-at-issue in the previous reexamination was owned by another company (later acquired by the petitioner) and, in response to an obviousness rejection, the other company had argued that the same cited references did not teach or suggest the blocking group. In the reexam proceedings, the examiner rejected that argument as unpersuasive, ultimately allowing the claims for a different reason. In the -00291 and related IPRs, the patent owner argued that the petitioner now contradicted that prior argument, asserting that this should be grounds for judicial estoppel. However, the Board determined that estoppel did not apply, in part, because the examiner in the reexamination did not adopt the other company’s argument as to what the cited reference discloses.

In Speedtrack, Inc. v. Endeca Techs., Inc., 524 Fed. Appx. 651, 659 (Fed. Cir. 2013), the Federal Circuit also refused to invoke judicial estoppel, stating that it was not convinced that the respondent was “successful” enough with its previous arguments for judicial estoppel to attach. It remains important that patent owners and petitioners are aware of the barriers to asserting judicial estoppel in such legal proceedings. This is one of many circumstances under which asserting judicial estoppel may be ineffective.


The -00291 and related IPR proceedings serve as a good reminder to revisit arguments, even failed ones, in the case history and related cases. However, even if a litigant advances a position that is inconsistent with their previously presented position during prosecution or a reexamination, judicial estoppel may not apply—particularly if their previous position was overcome or rejected by the examiner.