“…Furthermore, different knockout mouse models showed that impairment of ZP2 cleavage reduces fertility only partially, indicating that ZP2 processing is not essential to ensure monospermic fertilization and further successful development (Burkart et al, 2012; Gahlay et al, 2010; Rankin et al, 2003; Sachdev et al, 2012). The subfertility of these genetically modified female mice is likely to be caused by precocious ZP hatching and early embryonic loss (Burkart et al, 2012; Floehr, Dietzel, Schmitz, Chappell, & Jahnen‐Dechent, 2017; Winuthayanon et al, 2015) and there is no evidence of abnormal sperm accumulation in the perivitelline space of their oocytes (Gahlay et al, 2010; Rankin et al, 2003; Tokuhiro & Dean, 2018). The latter observation suggests the existence of a ZP2‐independent block to sperm penetration, which appears to depend on ovastacin's enzymatic activity based on the presence of supernumerary sperm in the perivitelline space of ovastacin‐deficient oocytes, as well as oocytes expressing ovastacin with an active‐site mutation (Tokuhiro & Dean, 2018).…”