Izumo sperm-egg fusion protein 4 is a sperm membrane protein which plays a key role in the fusion in the mouse. IZUMO4 has an N-terminal domain with significant homology to the N-terminal domain of Izumo.It belongs to the Izumo family . Izumo 4 is a soluble protein expressed in the testis and in other tissues. Izumo domain possesses the ability to form dimers, whereas the transmembrane domain or the cytoplasmic domain or both of Izumo 1 are required for the formation of multimers of higher order.