This is also the common testicular phenotype for carriers of complete AZFa deletions8,9 and, thus, this genotype-phenotype correlation provides substantial evidence that absence or impaired function of the DDX3Y protein is the underlying cause for SCO, azoospermia, and male infertility – independent whether the mechanism is an AZFa deletion or a point variant causing loss-of-function. This evidence concerns the gene USP9Y and male infertility.