In a female infant exhibiting cleft palate, hydrocephalus communicans, vitium cordis congenitum, and other anomalies, chromosome analysis revealed a C/D translocation. According to its replication pattern, the translocation chromosome comprised almost a whole X and the long arm of a chromosome 15. In accordance with the Lyon-hypothesis, in a number of the cells the translocated X, Xt, or the normal X, Xn, was the late-replicating chromosome. In a few cells both Xt and Xn were late labeled, which was interpreted as perhaps the result of somatic segregation. In some of the cells showing a late-replicating Xt, the autosomal segment, which is normally early replicating, was labeled; this observation was interpreted as a position effect. The genetic material lost comprised the product of the reciprocal translocation, consisting of the centromeric region of chromosome 15 and the telomeric region of the long arm of the X. The severe clinical anomalies of the patient were ascribed to the inactivation of the translocated autosomal segment observed in a part of the examined cells.