It is now known that SCID can be caused in humans by mutations in at least 10 different genes (Table 1) ( 6– 11), and the likelihood is that there are other causes yet to be discovered. Advances in molecular biology and the Human Genome Project as well as increased knowledge of various components of the immune system through studies of mutant mice and humans with genetically determined immunodeficiencies have all contributed to this understanding. Over the past 11 years, remarkable progress has been made in elucidating several other causes of this syndrome ( 5). However, it was not until 21 years later, in 1993, that a second fundamental cause of the condition was found, i.e., the molecular basis of X-linked human SCID ( 3, 4). The first discovered molecular cause of human SCID, adenosine deaminase deficiency, was reported in 1972 ( 2). In many families there was an X-linked recessive mode of inheritance while in others an autosomal recessive mode of inheritance was observed. This indicated that there was more than one cause for this fatal syndrome characterized by an absence of T cells and all adaptive immunity. In the ensuing years, differences were noted in inheritance patterns for SCID. Swiss infants with the condition were profoundly lymphopenic and died of infection before their first or second birthdays. Human SCID was first reported by Glanzmann and Riniker in 1950 ( 1).

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. 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. SCID, 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.

What gene is affected by scid