– Nonapeptide-1 (Melanostatine-5), a peptide hormone, is a selective antagonist of MC1R (Ki: 40 nM). Nonapeptide-1 is a competitive ?-MSH antagonist that potently inhibits intracellular cAMP and melanosome dispersion induced by ?-MSH in melanocytes (IC50: 2.5 nM and 11 nM, respectively). Nonapeptide-1 inhibits melanin synthesis, and can be used in the research of skin pigmentation and regulation of steroid production in the adrenal gland, skin cancer.