Section 5313 of Title 31 (with applicable regulations at 31 C.F.R. § 103.22), requires domestic financial institutions to report currency transactions which involve the payment, receipt, or transfer of United States coins or currency (or other monetary instruments as the Secretary of the Treasury may prescribe) in the amount of $10,000 or more. The report must be made on IRS Form 4789, commonly called a Currency Transaction Report (CTR), which is to be filed with the Internal Revenue Service within fifteen days following the day a reportable currency transaction occurs. Multiple cash transactions of under $10,000 apiece which occur in one day at one financial institution and aggregate over $10,000 must likewise be reported. It is important that prosecutors, in considering a CTR-related prosecution or forfeiture, consult the implementing regulations in effect during the period that the violations under consideration occurred.
[cited in JM 9-79.200]