Defensins are a family of microbicidal and cytotoxic peptides which are made by neutrophils and are thought to be involved in host defense. Defensins are abundant in the granules of neutrophils and are also found in the epithelia of mucosal surfaces such as those of the intestine, respiratory tract, urinary tract and vagina. There are six alpha-defensins, which are known as human neutrophil peptides (HNPs) in humans. HNP-1,2,3 and 4 are found in the microbicidal granules of neutrophils; HNP-5 and HNP-6 are located in Paneth cells of the intestinal tract. HNP-1 is found in the microbicidal granules of neutrophils and probably plays a role in phagocyte-mediated host defense. HNP-1, HNP-2 and HNP-3, which comprise about 30% of the total granule protein of the neutrophil, are secreted by most normal individuals. The HNP-1 protein is encoded by the a1-defensin (DEFA1) gene. HNP-3 is encoded by the a3-defensin (DEFA3) gene.