Human SSCs express KIT, BMPR2, and STRA8 during the initial stages of spermatogenesis, while KITLG expression in Sertoli cells increases in response to BMP4 stimulation.[19, 64, 76] Moreover, BMP4 and KITLG are abnormally low in men with a form of spermatogenic arrest known as non‐obstructive azoospermia (NOA), where Sertoli cells are characterized as immature.[77]. The gene discussed is BMP4; the disease is Obstructive azoospermia.