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Stages of mammalian fertilization. (1) Sperm swims towards the egg. (2) Sperm undergoes the acrosome reaction, in which the proteins in the acrosome are released and sperm is able to then cross the zona pellucida. (3) Sperm enters the perivitelline space and binds to the egg’s plasma membrane. (4) Sperm and egg become cytoplasmically contiguous. Asterisks indicate proteins that have previously been shown to be under positive selection. Underlined proteins are used in this study.
