SPEM2 and male infertility: As expected, Spem2-null sperm could not fertilize eggs by natural mating (Fig. 6A, B): Sperm from Spem2−/− mice yielded no (0%) two-cell embryos, whereas the average fertilization rate using sperm from wt mice was 87.9%, which further confirmed the male infertility phenotype of Spem2−/− mice.