37 CFR § 2.106 - Answer.
(a) If no answer is filed within the time initially set, or as may later be reset by the Board, the opposition may be decided as in case of default. The failure to file a timely answer tolls all deadlines, including the discovery conference, until the issue of default is resolved.
(1) An answer must be filed through ESTTA. In the event that ESTTA is unavailable due to technical problems, or when extraordinary circumstances are present, an answer may be filed in paper form. An answer filed in paper form must be accompanied by a Petition to the Director under § 2.146, with the fees therefor and the showing required under this paragraph (b).
(2) An answer shall state in short and plain terms the applicant's defenses to each claim asserted and shall admit or deny the averments upon which the opposer relies. If the applicant is without knowledge or information sufficient to form a belief as to the truth of an averment, applicant shall so state and this will have the effect of a denial. Denials may take any of the forms specified in Rule 8(b) of the Federal Rules of Civil Procedure. An answer may contain any defense, including the affirmative defenses of unclean hands, laches, estoppel, acquiescence, fraud, mistake, prior judgment, or any other matter constituting an avoidance or affirmative defense. When pleading special matters, the Federal Rules of Civil Procedure shall be followed. A reply to an affirmative defense shall not be filed. When a defense attacks the validity of a registration pleaded in the opposition, paragraph (b)(3) of this section shall govern. A pleaded registration is a registration identified by number by the party in the position of plaintiff in an original notice of opposition or in any amendment thereto made under Rule 15 of the Federal Rules of Civil Procedure.
(i) A defense attacking the validity of any one or more of the registrations pleaded in the opposition shall be a compulsory counterclaim if grounds for such counterclaim exist at the time when the answer is filed. If grounds for a counterclaim are known to the applicant when the answer to the opposition is filed, the counterclaim shall be pleaded with or as part of the answer. If grounds for a counterclaim are learned during the course of the opposition proceeding, the counterclaim shall be pleaded promptly after the grounds therefor are learned. A counterclaim need not be filed if the claim is the subject of another proceeding between the same parties or anyone in privity therewith; but the applicant must promptly inform the Board, in the context of the opposition proceeding, of the filing of the other proceeding.
(ii) An attack on the validity of a registration pleaded by an opposer will not be heard unless a counterclaim or separate petition is filed to seek the cancellation of such registration.
(iii) The provisions of §§ 2.111 through 2.115, inclusive, shall be applicable to counterclaims. A time, not less than thirty days, will be designated by the Board within which an answer to the counterclaim must be filed.
(iv) The times for pleading, discovery, testimony, briefs or oral argument may be reset or extended when necessary, upon motion by a party, or as the Board may deem necessary, to enable a party fully to present or meet a counterclaim or separate petition for cancellation of a registration.
(c) The opposition may be withdrawn without prejudice before the answer is filed. After the answer is filed, the opposition may not be withdrawn without prejudice except with the written consent of the applicant or the applicant's attorney or other authorized representative.