The process of mammalian X chromosome inactivation results in the inactivation of most, but not all, genes along one or the other of the two X chromosomes in females. On the human X chromosome, several genes have been described that "escape" inactivation and continue to be expressed from both homologues. All such previously mapped genes are located in the distal third of the short arm of the X chromosome, giving rise to the hypothesis of a region of the chromosome that remains noninactivated during development. The A1S9T gene, an X-linked locus that complements a mouse temperature-sensitive defect in DNA synthesis, escapes inactivation and has now been localized, in human-mouse somatic cell hybrids, to the proximal short arm, in Xp11.1 to Xp11.3. Thus, A1S9T lies in a region of the chromosome that is separate from the other genes known to escape inactivation and is located between other genes known to be subject to X inactivation. This finding both rules out models based on a single chromosomal region that escapes inactivation and suggests that X inactivation proceeds by a mechanism that allows considerable autonomy between different genes or regions on the chromosome.