26 C.F.R. § 20.2208-1
As used in this part, the term "citizen of the United States" is considered to include a decedent dying after September 2, 1958, who, at the time of his death, was domiciled in a possession of the United States and was a United States citizen, and who did not acquire his United States citizenship solely by reason of his being a citizen of such possession or by reason of his birth or residence within such possession. The estate of such a decedent is, therefore, subject to the tax imposed by section 2001. See paragraph (a)(2) of § 20.0-1 and § 20.2209-1 for further information relating to the application of the Federal estate tax to the estates of decedents who were residents of possessions of the United States. The application of this section may be illustrated by the following example and the examples set forth in § 20.2209-1 :
Example. A, a citizen of the United States by reason of his birth in the United States at San Francisco, established residence in Puerto Rico and acquired a Puerto Rican citizenship. A died on September 4, 1958, while a citizen and domiciliary of Puerto Rico. A's estate is, by reason of the provisions of section 2208, subject to the tax imposed by section 2001 inasmuch as his United States citizenship is based on birth in the United States and is not based solely on being a citizen of a possession or solely on birth or residence in a possession.
26 C.F.R. §20.2208-1