35 U.S.C. § 151

1. (a)If it appears that an applicant is entitled to a patent under the law, a written notice of allowance of the application shall be given or mailed to the applicant. The notice shall specify a sum, constituting the issue fee and any required publication fee, which shall be paid within 3 months thereafter.
2. (b)Upon payment of this sum the patent may issue, but if payment is not timely made, the application shall be regarded as abandoned.