The in vivo fertilization rate calculated as the percentage of the 2-cell embryos was 82.9% in wt mice and 4.1% in mutant mice (Fig. 3a), suggesting that the infertility of Prss55−/− males was mainly due to the fertilization defects but not due to the developmental disability of the embryos. The gene discussed is PRSS55; the disease is Infertility.