The following is taken from the ARRL Letter, dated 2009-12-11:

"If you're an American ham, chances are that your call sign was issued by the Federal Communications Commission. A "no brainer," right? Well, if you're an American ham who happens to be stationed at Guantanamo Bay or at one of the US bases in the Antarctic, your call sign is not issued by the FCC -- it's issued by the base commander. Guantanamo Bay (or Gitmo as it's commonly called) uses the KG4 prefix, followed by a two-letter suffix; this block is reserved exclusively for American hams at Gitmo. As for Antarctica, the Antarctic Treaty, signed on December 1, 1959 (and entered into force on June 23, 1961), established the legal framework for the management of Antarctica, including allocation of amateur call signs; the National Science Foundation received their block on July 1, 1959. US military hams in Japan and Korea are also issued special call signs:

Related article
Federal Communication Commission Call Sign Systems - Call Sign Availability

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