Cathelicidins are a major family of antimicrobial peptides present in vertebrate animals with potent microbicidal and immunomodulatory activities.Four cathelicidins,namely fowlicidins 1 to 3 and cathelicidin B1,have been identified in chickens.As a first step to understand their role in early innate host defense of chickens,we examined the tissue and developmental expression patterns of all four cathelicidins.Real-time PCR revealed an abundant expression of four cathelicidins throughout the gastrointestinal,respiratory,and urogenital tracts as well as in all primary and secondary immune organs of chickens.Fowlicidins 1 to 3 exhibited a similar tissue expression pattern with the highest expression in the bone marrow and lung,while cathelicidin B1 was synthesized most abundantly in the bursa of Fabricius.Additionally,a tissue-specific regulatory pattern was evident for all four cathelicidins during the first 28 days after hatching.The expression of fowlicidins 1 to 3 showed an age-dependent increase both in the cecal tonsil and lung,whereas all four cathelicidins were peaked in the bursa on day 4 after hatching,with a gradual decline by day 28.An abrupt augmentation in the expression of fowlicidins 1 to 3 was also observed in the cecum on day 28,while the highest expression of cathelicidin B1 was seen in both the lung and cecal tonsil on day 14.Collectively,the presence of cathelicidins in a broad range of tissues and their largely enhanced expression during development are suggestive of their potential important role in early host defense and disease resistance of chickens.