DEFB119 (defensin beta 119) is a beta-defensin gene located on chromosome 20.1 that functions as a physiological sperm chemoattractant and possesses antimicrobial activity. Primary function: DEFB119 serves as a critical mediator of sperm chemotaxis, with the female reproductive tract epithelium and cumulus-oocyte complex secreting DEFB119 to guide sperm toward the egg 1. Mechanism: The protein elicits calcium mobilization via the CatSper channel in capacitated sperm, inducing directional chemotaxis 1. DEFB119 belongs to a diverse family of cationic antimicrobial peptides characterized by six conserved cysteine residues 2. The gene is differentially regulated by androgens and shows expression restricted to the male reproductive tract 2. Disease relevance: Exon mutations in DEFB119 have been identified in women with idiopathic infertility, correlating with reduced follicular fluid DEFB119 levels 1. Clinical significance: DEFB119 levels in follicular fluid correlate with chemotactic potency and predict infertile outcomes, suggesting potential diagnostic applications for infertility assessment 1. Additionally, DEFB119 has been identified as a potential drug target for lung cancer through proteome-wide Mendelian randomization analysis 3.