A 13-year-old girl with short stature and obesity was found to have 46, XXq-in 90% of cultured leukocytes and 45, X in 10%. Autoradiographic studies showed the long-arm deleted X to be the late replicating X chromosome. In no cell was the normal X chromosome found to be late replicating. Our findings, combined with others from the literature, imply genetic inactivation of all structurally abnormal X chromosomes. Therefore, a previous hypothesis regarding the location of the Xga gene on the short arm of the X chromosome is not valid. The hypothesis that normal somatic development in females may require the presence of the short arm of both X chromosomes prior to X inactivation remains likely. The XO line found in our patient probably accounts for her short stature.