This is a family of unknown function. Family members include the archaeal homolog the bacterial RelB, a toxin-antitoxin system which is activated during amino acid starvation. This family is mostly found in thermococcus archaea [1].

1: Crystal structure of archaeal toxin-antitoxin RelE-RelB complex with implications for toxin activity and antitoxin effects. Takagi H, Kakuta Y, Okada T, Yao M, Tanaka I, Kimura M; Nat Struct Mol Biol. 2005;12:327-331. PMID:15768033