Klinefelter's syndrome in nontwin brothers and maternal XX/XXX mosaicism

A case of two nontwin brothers, 19 and 17 years old, who had both Klinefelter's syndrome with a chromosomal mosaicism 46 XY/47 XXY, is reported here. The analysis of their mother's karyotype revealed a 46 XX/47 XXX mosaicism. It is hypothesized that the presence of an extra X chromosome in all three subjects could depend on the transmission of two X chromosomes from the mother to the sons or, less likely, on an increased liability to nondisjunction of the X chromosomes during one of the early mitotic divisions in the zygotes.