PIWIL1 and Azoospermia: Several germ cell markers have been investigated and used in reproductive biological and biotechnological studies on vertebrates, including DEAD (Asp-Glu-Ala-Asp) box polypeptide 4 (ddx4; previously known as vasa), microRNA-mediated repression inhibitor 1 (dnd1; previously known as dnd), nanos protein (nanos3; previously known as nanos), piwi-like RNA-mediated gene silencing 1 (piwil1; previously known as piwi), sry-box transcription factor sox-3 (sox3), and deleted in azoospermia-like (dazl) [5,6,7,8,9,10,11,12].