The invention provides a method of diagnosing X-linked severe combined immunodeficiency (XSCID) in males or determining whether females are carriers. This method is based upon the presence of either a mutated or truncated IL-2Rgamma gene. The invention also discloses a method of treating XSCID as well as a method for monitoring the therapy. Lastly, the invention provides a promoter which regulates the expression of IL-2Rgamma, a vector comprising a DNA molecule operably linked to the promoter, a cell host that has been transformed with the vector, and a transgenic mouse comprising the promoter or a mutant IL-2Rgamma.