SPEM1 and Infertility: Studies by Zheng et al., 2007, discovered that the gene spermatid maturation 1 (Spem1) is testis-specific in mice and humans, and inactivation in mice leads to defective shedding of cytoplasm during spermiogenesis, as such, the sperm heads are malformed and positioned incorrectly for fertilization, thus leading to infertility.