In mouse the melanocortin 5 receptor is known to regulate sebaceous gland function. To clarify its role in man, we have studied melanocortin 5 receptor expression in skin, and allelic variation at the melanocortin 5 receptor locus in diverse human populations and candidate disease groups. Melanocortin 5 receptor protein and mRNA expression were studied by immunohistochemistry and reverse transcriptase polymerase chain reaction. Melanocortin 5 receptor mRNA was detected in normal skin and cultured keratinocytes but not in cultured fibroblasts or melanocytes. Immunohistochemistry revealed melanocortin 5 receptor immunoreactivity in the epithelium and appendages, including the sebaceous gland, eccrine glands, and apocrine glands, as well as low level expression in the interfollciular epidermis. In order to screen for genetic diversity in the melanocortin 5 receptor that might be useful for allelic association studies we sequenced the entire melanocortin 5 receptor coding region in a range of human populations. One nonsynonymous change (Phe209Leu) and four synonymous changes (Ala81Ala, Asp108Asp, Ser125Ser, and Thr248Thr) were identified. Similar results were found in each of the populations except for the Inuit in which only the Asp108Asp variant was seen. The apparent "global distribution" of melanocortin 5 receptor variants may indicate that they are old in evolutionary terms. Variation of melanocortin 5 receptor was examined in patients with acne (n = 21), hidradenitis supprativa (n = 4), and sebaceous gland lesions comprising sebaceous nevi, adenomas, and hyperplasia (n = 13). No additional mutations were found. In order to determine the functional status of the Phe209Leu change, increase in cAMP in response to stimulation with alpha-melanocyte-stimulating hormone was measured in HEK-293 cells transfected with either wild-type or the Phe209Leu variant. The variant melanocortin 5 receptor was shown to act in a concentration-dependent manner, which did not differ from that of wild type. We have therefore found no evidence of a causative role for melanocortin 5 receptor in sebaceous gland dysfunction, and in the absence of any association between variation at the locus and disease group, the pathophysiologic role of the melanocortin 5 receptor in man requires further study.