Severe Combined Immunodeficiency (SCID) is a rare primary immunodeficiency disease often characterized by a block in T cell development, which may also affect the normal development of B cells and NK cells. Several different mutations are known to give rise to SCID, and multiple genes are involved. Consequently, there are several different forms of SCID, which can be classified according to the metabolic and cellular defects that impede normal lymphocyte function. The two most prevalent forms of SCID are X-linked SCID and adenosine deaminase (ADA) deficiency SCID, together accounting for approximately 70-80% of disease cases. Other genetic abnormalities associated with this syndrome range from defective T cell receptor rearrangement to non-functional signaling molecules. Recently, a new genetic defect has been described in which mutations in a key component of Ca(2+) release activated-channels (CRAC) result in T lymphocyte malfunction.