To obtain in vivo developed human spermatogonia, human testicular samples were biopsied from obstructive azoospermia (OA) patients, and the existence of undifferentiated spermatogonia in these samples was confirmed by immunohistofluorescence (IHF) with an antibody against a SSC marker GFRα1 (Figure 1A). The gene discussed is GFRA1; the disease is Obstructive azoospermia.