In support of the cellular interference hypothesis came the DEE9 diagnosis for some male patients with a mosaic expression of PCDH19, due to a post-zygotic somatic mutation in their unique PCDH19 allele [68,71,72,73,74,75,76,77] or to the mutation in one of the two PCDH19 alleles in the context of Klinefelter syndrome (XXY) [78]. Here, PCDH19 is linked to Klinefelter syndrome.