IZUMO1, a sperm-specific protein, is essential for sperm-egg plasma membrane binding and fusion. It is involved in the fertilization process and in the fusion of the sperm with the egg (1,2). IZUMO1 is expressed as at least four isoforms produced by alternative splicing and contains one Ig-like (immunoglobulin-like) C2-type domain (3). Lack of IZUMO1 in males, due to homozygous mutation in the gene that encodes IZUMO1, results in sterility due to an inability to penetrate the plasma membrane of the egg (4).