Discussions of X-chromosome inactivation include the role of specific genes in the creation or maintenance of the inactive state—for example, XIST (MIM *314670), TSIX (MIM *300181), and XCE (MIM *300074). Familial skewed X inactivation (MIM #300087) is sometimes due to mutations in the XIST gene; skewed X inactivation occurs also in women heterozygous for some X-linked disorders, including X-linked severe combined immunodeficiency (MIM #300400), Wiskott-Aldrich syndrome (MIM #301000), and dyskeratosis congenita (MIM #305000), in which cells with the mutation-carrying X chromosomes are at a selective disadvantage. In such instances, the skewing can be used as a method for diagnosing the heterozygous carrier state. In some instances, skewed inactivation may be a main piece of evidence establishing X-linked inheritance; for example, Aicardi syndrome (MIM %304050) is probably X-linked dominant with male lethality, to judge from the finding of skewed X inactivation in females.