Physical mapping of loci in the distal half of the short arm of the human X chromosome: Implications for the spreading of X-chromosome inactivation

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Abstract
The relative order of 11 loci in the distal half of the short arm of the human X chromosome was examined using a panel of somatic cell hybrids containing structurally rearranged X chromosomes. The results show that the gene for phosphoribosylpyrophosphate synthetase 2 (PRPS2) is located betweenZFX (zinc finger protein, X-linked) andSTS (steroid sulfatase). The results also confirm the localization ofZFX distal toPOLA (α-DNA polymerase). Previous studies have shown thatSTS andZFX escape X-inactivation whereasPOLA undergoes inactivation. Evaluation ofPRPS2 expression in somatic cell hybrids containing inactive human X chromosomes showed thatPRPS2 undergoes X-inactivation. These results provide further evidence for interspersion of loci that do and do not undergo X-inactivation on the human X chromosome.