GLIPR1L1 is a testis-enriched protein required for optimal male fertility, particularly at the stage of sperm-oocyte interaction. The protein is a member of the CAP (cysteine-rich secretory protein, antigen 5, and pathogenesis-related 1) superfamily and functions in sperm-zona pellucida binding 1. During spermatogenesis, GLIPR1L1 undergoes N-linked glycosylation and localizes to the connecting piece and sperm head, with redistribution to anterior sperm head regions following capacitation 1. The protein plays a role in optimizing acrosome function and facilitating IZUMO1 translocation during the acrosome reaction, contributing to sperm-egg fusion [UniProt]. GLIPR1L1 is also a component of epididymosomes and CD9-positive microvesicles that mediate lipid and protein transfer to spermatozoa during epididymal maturation. Like other GLIPR1-family members, GLIPR1L1 is a p53 target gene 2, suggesting potential roles in stress response. Despite its reproductive specialization, knockout studies demonstrate that GLIPR1L1 is individually dispensable for male fertility in mice, with null males showing normal fecundity, sperm morphology, and motility 3. However, the gene's enrichment in male germ cells makes it a potential biomarker for sperm quality assessment.