Highly glycosylated atypical beta-defensin involved in several aspects of sperm function. Facilitates sperm transport in the female reproductive tract and contributes to sperm protection against immunodetection; both functions are probably implicating the negative surface charge provided by its O-linked oligosaccharides in the sperm glycocalyx. Involved in binding of sperm to oviductal epithelial cells to form a sperm reservoir until ovulation. Release from the sperm surface during capacitation and ovaluation by an elevation of oviductal fluid pH is unmasking other surface components and allows sperm to penetrate the cumulus matrix and bind to the zona pellucida of the oocyte (By similarity). In vitro has antimicrobial activity and may inhibit LPS-mediated inflammation (PubMed:19373462, PubMed:23229569). Homodimer or homooligomer; disulfide-linked. Secreted by epididymal cells and is absorbed to the surface of sperm during transit through the epididymis (By similarity). Mainly located on the sperm acrosome. High-level and epididymis-specific expression. Expression is down-regulated in infertile men. O-glycosylated; glycans contain alpha(2,3)-linked sialic acids. May be involved in infertility. Homozygosity for frameshift truncating mutations are associated with reduced sperm O-linked glycan content, impaired sperm mobility and a reduced live birth rate (PubMed:21775668, PubMed:25721098). However, for one common mutation the change in sperm sialic acid levels has been challenged (PubMed:26832966). Belongs to the beta-defensin family.