Accordingly, a study describing a male with Klinefelter syndrome (47, XXY) and a large CDKL5 COOH-truncation presenting a phenotype comparable to those of other CDKL5-positive boys suggests that the presence of a wild-type CDKL5 allele and a balanced pattern of X-inactivation does not reduce the severity of the disease [20]. This evidence concerns the gene CDKL5 and Klinefelter syndrome.