A 21-month-old boy with multiple congenital anomalies was found to have 49 chromosomes with an XXXXY sex chromosome constitution. He was mentally retarded with skeletal abnormalities, hypertelorism, malformed and asymmetrical ears, a short broad neck and small external genitalia. A testicular biopsy specimen showed defective germ cell and tubule development. Autoradiographic studies with tritiated thymidine showed asynchrony in DNA synthesis of the chromosomes of cultured skin and testis cells. In cells labelled late during the DNA synthetic period three heavily labelled X chromosomes were demonstrated. Autoradiographs of interphase nuclei derived from the same cultures showed localized labelling of heterochromatic foci, including the sex chromatin, in a small percentage of cells. The results are interpreted as evidence for the derivation of a sex chromatin body from an entire X chromosome which replicates late during the DNA synthetic period.