IZUMO4 is a testis-enriched gene encoding a ~25 kDa protein localized to the acrosomal region of sperm 1. The primary function of IZUMO4 involves participation in sperm-oocyte interaction during fertilization. Mechanistically, IZUMO4 proteins are immunodetected in sperm extracts and localize to both acrosome-intact and acrosome-reacted cells 1. Blocking IZUMO4 with specific antibodies significantly reduces fertilization outcomes, including lower zona pellucida penetration (CPA), hemizona binding (HZA), and sperm-perivitelline space interaction (SPA) rates 1. Protein modeling suggests IZUMO4 interacts with oocyte JUNO, essential for gamete fusion 1. Regarding clinical significance, IZUMO4 transcript levels are significantly reduced in teratozoospermic men 1, indicating potential diagnostic value as a biomarker for male infertility. Notably, IZUMO4 knockout mice display normal male fertility, suggesting the gene is individually dispensable for reproduction in mice 2. However, IZUMO4 dysregulation has been observed in smoking males post-COVID-19 infection, correlating with impaired sperm function 3, suggesting environmental and pathogenic factors may impact IZUMO4 expression and male reproductive health.