Are there any restrictions on who can file for ex parte reexamination?

Yes, there is one specific restriction mentioned in MPEP 2212:

The only ‘person’ who is barred from filing a request for ex parte reexamination of a patent under 35 U.S.C. 302 is one who is barred from doing so by the estoppel provisions of AIA 35 U.S.C. 315(e)(1) or 35 U.S.C. 325(e)(1) based on inter partes review and post grant review, respectively, once the estoppel attaches.

This means that if a person has been involved in certain types of patent review proceedings (inter partes review or post grant review) and is subject to estoppel as a result, they may be barred from filing an ex parte reexamination request.

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Topics: MPEP 2200 - Citation Of Prior Art And Ex Parte Reexamination Of Patents, MPEP 2212 - Persons Who May File A Request For Ex Parte Reexamination Under 35 U.S.C. 302, Patent Law, Patent Procedure
Tags: Estoppel, ex parte reexamination, inter partes review, patent law, post grant review, USPTO