¡Ò-defensins (also designated BDs, or hBDs in human) are small cationic peptides with broad-spectrum antimicrobial activity against a variety of enveloped viruses, fungi and bacteria. Produced in mucosal epithelia and neutrophils of several species, ¡Ò-defensins are developmentally regulated. The family of ¡Ò-defensin proteins share a common defensin-motif that is characterized by multiple cysteine residues and a highly conserved tertiary structure. Besides playing a significant role in host immune defense, many ¡Ò-defensins also are involved in sperm maturation and capacitation. ¡Ò-defensin 121 is a 76 amino acid secreted protein that most likely contains a signal peptide sequence that requires cleavage by proteolytic enzymes in order to become biologically active. While expressed at low levels in testis, ¡Ò-defensin 121 is abundantly expressed in epididymal caput and corpus and, unlike other ¡Ò-defensins, it does not appear to be regulated by androgens.