19 U.S.C. § 537: Officers, informers, and defendants as witnesses

No officer, or other person entitled to or claiming compensation under any provision of Act June 22, 1874 (chapter 391, 18 Statutes 188) shall be thereby disqualified from becoming a witness in any action, suit, or proceeding for the recovery, mitigation, or remission thereof, but shall be subject to examination and cross-examination in like manner with other witnesses, without being thereby deprived of any right, title, share, or interest in any fine, penalty, or forfeiture to which such examination may relate; and in every such case the defendant or defendants may appear and testify and be examined and cross-examined in like manner.