MPEP § fp29.100
Applicant is reminded that any reply to this communication must be signed either by a patent practitioner (i.e., a patent attorney or agent registered to practice before the United States Patent and Trademark Office) or by the applicant. If the applicant is a juristic entity, the reply must be signed by a patent practitioner. See 37 CFR 1.33(b).
Add this paragraph at the end of a notification of refusal or other Office action where a patent attorney or agent registered to practice before the United States Patent and Trademark Office has not been appointed as applicant's representative. If this paragraph was included in a prior Office action, it may be, but is not required to be, included in a subsequent Office action.