Although both IZUMO and FLOT2 change their location after the acrosome reaction, they do not seem to colocalize. While FLOT2 is restricted to the equatorial segment, IZUMO can move farther, crossing to the ventral para-acrosomal sperm domains. These compartments are separated by a specialized structure that seems to be defined during epididymal maturation and involves tyrosine phosporylation [54] . Other proteins reported to redistribute after the acrosome reaction are localized to the equatorial segment and are not able to pass the postacrosomal boundary [52, 53] . This suggests that protein redistribution after the acrosome reaction is a regulated process that may comprise a particular mechanism for IZUMO. It is a matter to be resolved why only IZUMO among all the proteins that move after the acrosome reaction is able to cross the postacrosomal boundary and whether this has a functional significance considering its relevance in sperm-egg fusion [55] .