37 CFR 1.31 : Patents, Trademarks, And Copyrights (§ 1.1 To Chapter V) — (§ 1.1 To Chapter V) — United States Patent And Trademark Office, Department Of Commerce (§ 1.1 To Parts 151-199) — General (§ 1.1 To §§ 5.31-5.33) — Rules Of Practice In Patent Cases (§ 1.1 To § 1.1071) — National Processing Provisions (§ 1.31 To § 1.378) — Prosecution Of Application And Appointment Of Attorney Or Agent (§ 1.31 To § 1.36) — Applicant may be represented by one or more patent practitioners or joint inventors.

4 Aug 2016

An applicant for patent may file and prosecute the applicant's own case, or the applicant may give power of attorney so as to be represented by one or more patent practitioners or joint inventors, except that a juristic entity (e.g., organizational assignee) must be represented by a patent practitioner even if the juristic entity is the applicant. The Office cannot aid in the selection of a patent practitioner.

37 CFR 1.31
[77 FR 48813, Aug. 14, 2012]