Most often, this disease affects males whose mother is a carrier (heterozygous) for the disorder. Because females have two X-chromosomes, the mother will not be affected by carrying only one abnormal X-chromosome, but any male children will have a 50% chance of being affected with the disorder by inheriting the faulty gene. Likewise, her female children will have a 50% chance of being carriers for the immunodeficiency. X-SCID can also arise through de novo mutations and can be prevented in females by X-inactivation. In X-inactivation the preferential selection of the non-mutant X chromosome during development results in the outcome that none of the mature female cells actively express the X-SCID mutation, they are immunologically unaffected and have no carrier burden. A de novo mutation is an alteration in a gene caused by the result of a mutation in a germ cell (egg or sperm) or in the fertilized egg itself, rather than having been inherited from a carrier. Since only 1/3 of all X-SCID patients have a positive family history of SCID, it is hypothesized that de novo mutations account for a significant percentage of cases. X-inactivation occurs in a completely random manner, in females, very early in embryonic development. Once an X is inactivated, it remains inactivated throughout the life of that cell and any of its daughter cells. It is important to note that X-inactivation is reversed in female germline cells, so that all new oocytes receive an active X. Regardless of which X is inactivated in her somatic cells, a female will have a 50% chance of passing on the disease to any male children. Interleukins are produced by lymphocytes, among other cell types, and are released in response to antigenic and non-antigenic stimuli. The gene IL2RG codes for the common gamma chain protein, which is a common subunit of the individual receptors for Interleukin 2, Interleukin 4, Interleukin 7, Interleukin 9, Interleukin 15 and Interleukin 21. Signalling from these receptors normally promotes growth and differentiation of T-cells, B cells, natural killer cells, glial cells, and cells of the monocyte lineage, depending on the cell type and receptor activated. The most important receptors for X-SCID are those for Interleukin 2, Interleukin 4, Interleukin 7, and Interleukin 15.