GDNF and Azoospermia: We have recently revealed distinct expression of SCF, BMP4, and GDNF in human Sertoli cells between non-obstructive azoospermia (NOA) patients with impaired spermatogenesis and obstructive azoospermia (OA) patients with normal spermatogenesis [18], suggesting that there might exist an association of these factors secreted by Sertoli cells and male infertility.