BMP4 and Azoospermia: We have recently revealed that SCF, BMP4, and GDNF are differentially expressed in human Sertoli cells between NOA patients and OA patients with normal spermatogenesis [13] and that BMP4 promotes the proliferation of human Sertoli cells through the Smad1/5 and ID2/3 pathway [14], which provides novel insights into genetic etiology of NOA azoospermia.