It further complicates the separation of the clusters that cells have a mixed cellular expression of many of the genes that are specific for spermatogenesis e.g. a relatively low expression that initiates in spermatogonia that is followed by a large increase in expression in spermatocytes (for example Deleted in azoospermia-like (Dazl) [22]). This evidence concerns the gene DAZL and Azoospermia.