Melatonin receptor expression has been identified in the retinas of several species. The major types of melatonin receptors that have been cloned are members of the superfamily of G-protein-coupled receptors. Melatonin receptors have been classified as Mel1a, Mel1b, and Mel1c subtypes. These three subtypes are expressed in tissues, including the retinas, of lower vertebrates such as amphibians, fish, and birds. Mammalian melatonin receptors are classified according to their homology to the nonmammalian receptors, and according to their pharmacological properties.