Defensins are widely distributed and broad-spectrum antimicrobial peptides with activities against bacteria, fungi, and enveloped viruses. Defensins have been isolated from granules of neutrophils from humans, rabbits, rats, and guinea pigs. They have also been found in lung macrophages as well as in Paneth cells of the human, rabbit, and mouse small intestine. The human beta-defensin-2 was recently isolated from human skin. In this study, we detected the expression of mRNA for the defensin cryptdin in BALB/c mouse skin by means of reverse transcriptase PCR amplification. Expression was also detected in dispase-separated epidermis and cultured keratinocytes, but expression was not detected in fibroblasts. The expression of cryptdin mRNA was found to begin on embryonic day 17.5. As determined with specific primers, the cDNA sequence cloned from the skin was found to be identical to that previously reported for cryptdin-5. cDNA derived from cultured keratinocytes demonstrated the sequences of the cryptdin-6 and cryptdin-1 isoforms. In situ hybridization analysis showed that the mRNA of cryptdin was expressed in the suprabasal keratinocytes of the skin in embryonic and neonatal days and then shifted to the hair bulbs in the skin of adult mice.