DDX4 and Azoospermia: Although it is reported that a series of genes are involved in the process of gonadal differentiation and development, only some of them have been identified so far, such as nanos3, vasa (Ddx4, DEAD box polypeptide 4), dazl (deleted in azoospermia-like genes), and piwi (a P-element-induced wimpy testis) [5,6,7].