http://www.creativebiomart.net/description_382731_12.htm

IZUMO1 has an N-terminal domain with significant homology to the N-terminal domain of Izumo. Izumo is a sperm membrane protein which plays a key role in the fusion in the mouse. It has an Immunoglobulin (Ig) domain and an N-terminal domain for which neither the functions nor homologous sequences are known. Besides IZUMO1, there are other 3 proteins which have an N-terminal domain with significant homology to the N-terminal domain of Izumo. They are Izumo 2, 3, and 4. Izumo 1-3 are transmembrane proteins expressed specifically in the testis, and Izumo 4 is a soluble protein expressed in the testis and in other tissues. Izumo 1, 3, and 4 formed protein complexes on sperm, Izumo 1 forming several larger complexes and is involved in the fertilization process. It also may be involved in the fusion of the sperm with the egg.