Early annotation suggested this family, SepSecS, of several eukaryotic and archaeal proteins, was involved in antigen-antibodies responses in the liver and pancreas. Structural studies show that the family is O-phosphoseryl-tRNA(Sec) selenium transferase, an enzyme involved in the synthesis of the amino acid selenocysteine (Sec). Sec is the only amino acid whose biosynthesis occurs on its cognate transfer RNA (tRNA). SepSecS catalyzes the final step in the formation of the amino acid. The early observation that autoantibodies isolated from patients with type I autoimmune hepatitis targeted a ribonucleoprotein complex containing tRNASec led to the identification and characterization of the archaeal and the human SepSecS. SepSecS forms its own branch in the family of fold-type I pyridoxal phosphate (PLP) enzymes that goes back to the last universal common ancestor which explains why the archaeal sequences spcS and MK0229 are annotated as being pyridoxal phosphate-dependent enzymes.