SPACA4 is a testis-specific, spermatogenic cell protein essential for mammalian fertilization 1. Expressed during early spermatogenesis and localized to the acrosome, SPACA4 functions as a glycosylphosphatidylinositol (GPI)-anchored membrane protein that mediates sperm-zona pellucida binding 12. Antibody inhibition studies demonstrate that SPACA4 is required for efficient sperm binding to the zona pellucida, though it is lost during the acrosome reaction 1. Notably, SPACA4 represents the tetrapod homolog of the fish oocyte protein Bouncer, which is sufficient for species-specific fertilization, suggesting evolutionary conservation of SPACA4's reproductive role 3. In clinical infertility contexts, SPACA4 is significantly downregulated in men with non-obstructive azoospermia compared to controls, identifying it as a potential biomarker for spermatogenic impairment 45. However, prospective clinical studies found no association between SPACA4 protein expression levels and fertilization rates, indicating its role operates in a non-dose-dependent manner 1. SPACA4's narrow substrate specificity among GPI-anchored proteins suggests specialized function in male fertility 2.