S CID, a syndrome characterized by the absence of T cells and adaptive immunity, can result from mutations in multiple genes that encode components of the immune system. Three such components are cytokine receptor chains or signaling molecules, five are needed for antigen receptor development, one is adenosine deaminase — a purine salvage pathway enzyme, and the last is a phosphatase, CD45. In this issue of the JCI, a report describes how complete deficiency of the CD3ε chain of the T cell antigen receptor/CD3 complex causes human SCID.