We have studied a family with an X/X translocation which was found in a 16-year-old daughter and in her 34-year-old mother. The daughter had short stature, secondary amenorrhea, and many Turner’s stigmata. The mother, 5-ft tall, had a few Turner’s stigmata and had developed secondary amenorrhea at age 24. Both peripheral leukocyte and skin fibroblast cultures of the daughter revealed, by conventional karyotype analysis, 46 chromosomes with one C-group chromosome missing and with an extra No. 2-like chromosome in all metaphase cells examined. By both Q- and G-banding techniques, this abnormal karyotype was identified as 46,X, + t(X;X)(p22;ql3). An identical translocation chromosome was demonstrated in the mother’s cells, but 50 % of the cells had a 45,X chromosomal constitution. Autoradiographic studies showed that the X/X translocation chromosome was the late-replicating X in all cells examined from both the daughter and the mother. The difference in the phenotypic manifestations of the daughter and of her mother can be explained perhaps by the different contents of the other active X chromosomes. When the same X chromosome is inactivated in all cells, mutations present in the active X chromosome are not compensated and therefore are expressed.