26 C.F.R. § 31.3121(b)(5)-1
Services performed in the employ of an instrumentality of the United States are excepted from employment if such instrumentality is exempt from the employer tax imposed by section 3111 by virtue of any other provision of law which specifically refers to such section 3111 or the corresponding section of prior law (section 1410 of the Internal Revenue Code of 1939) in granting exemption from the employer tax. This exception does not operate to exclude from employment services performed in the employ of an instrumentality of the United States unless the Congress has granted to such instrumentality a specific exemption from the tax imposed by section 3111 or the corresponding section of prior law. For provisions which make general exemptions from Federal taxation ineffectual as to the employer tax imposed by section 3111, see § 31.3112-1 . For other exceptions from employment applicable with respect to services performed in the employ of an instrumentality of the United States, see § 31.3121(b)(6) -1.
26 C.F.R. §31.3121(b)(5)-1