CLCN4 and Klinefelter syndrome: However, we do note that this unaffected mother had a mosaic karyotype (47,XXX/46,XX) and it is possible that the ‘extra’ X chromosome may have somewhat moderated her phenotypic expression, as we considered for the unaffected male with Klinefelter syndrome, with an inherited CLCN4 variant which resulted in a severe phenotype in his male relatives [5].