SPINK2 and Azoospermia: However, we claim that also heterozygous variants can be causative, similar to SPINK2 whose deficiency has been documented as responsible for oligoasthenozoospermia in heterozygous mice versus azoospermia in homozygous animals, as well as azoospermia in human homozygous brothers versus their heterozygous father with oligozoospermia (Kherraf et al., 2017).