X-linked severe combined immunodeficiency, or X-SCID, is a chromosomal disorder in which the immune system lacks multiple protective cells that defend the body from disease. As of 2024, approximately one in 75,000 males have X-SCID. X-SCID is the most common type of SCID, which encompasses a range of disorders that all involve defects in immune cells that fight infections, leaving the individual susceptible to life-threatening diseases. X-SCID, which typically only affects males, arises due to a mutation in the interleukin 2 receptor gamma chain, or IL-2RG, gene on the X chromosome. IL-2RG aids certain immune cells to develop their protective functions, so a mutation in the receptor results in a dysfunctional immune system. Doctors most commonly use bone marrow transplants to treat X-SCID. By studying cases of X-SCID, researchers more clearly defined the role of lymphocytes in immune system development and overall disease protection. Unless detected and treated early, the defect in immune cells from X-SCID makes the individual prone to severe, recurrent infections, which are almost always fatal.