X chromosome: A sex chromosome found in both females and males. Normal females usually have two X chromosomes. Normal males usually have one X along with a Y chromosome. The complete chromosome complement consisting of 46 chromosomes is conventionally written as 46,XX for females and 46,XY for males.

The X chromosome carries hundreds of genes. Only a minority of the genes on the X have anything to do with sex. This is consistent with the concept that the X and Y chromosomes evolved from an ancient pair of autosomal (nonsex) chromosomes and that the X chromosome retains much of its old autosomal information.

The inheritance of genes on the X follows special rules because males have only a single X chromosome, almost no gene on the X has a counterpart on the Y, and genes on the X are expressed in males. Genes inherited in this manner are said to be sex-linked or, more precisely, X-linked.

Examples of the genes on the X chromosome include those for the enzyme steroid sulfatase, episodic muscle weakness, ocular albinism, Charcot-Marie-Tooth disease (one kind of this neurologic disease), retinoschisis (retinal splitting due to degeneration), infantile spasm syndrome, agammaglobulinemia (a form of immune deficiency), Aicardi syndrome, Duchenne and Becker muscular dystrophy, chronic granulomatous disease, hemophilia A, hemophilia B, and red-green colorblindness.