SG-21 was the twenty-first SG team formed at Stargate Command. It was created sometime before 2004. (SG1: "Death Knell")
They were sent, together with SG-11 and members of SG-1, to track down the Kull warrior that wiped out Earth's Alpha Site. (SG1: "Death Knell")
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- GateWorld's article on SG-21 in The Stargate Omnipedia
- SG-21 article on Stargate SG-1 Solutions' The StargateWiki