Protease inhibitor required for egg fertilization. Required to prevent premature zona pellucida hardening before fertilization, probably by inhibiting the protease activity of ASTL, a protease that mediates the cleavage of ZP2 and triggers zona pellucida hardening. Liver, lung and tongue. Female infertility due to a block early in fertilization. Oocytes undergo premature zona pellucida hardening. Belongs to the fetuin family.