“…Nowadays, functional MTRs have been cloned from various species, such as human, rat, mouse, sheep, rabbit, hamster, chicken, fish, xenopus, platypus etc. 15,16 Most recent alignment of sequences from 300 vertebrates revealed the likely existence of four ancestral vertebrate MTR subtypes named Mel1a, Mel1b, Mel1c and Mel1d, with Mel1d being closely related to Mel1a and Mel1c to Mel1b. 17 While Mel1d was lost in mammals and birds, Mel1c evolved into GPR50 in eutherian mammals and lost its high-affinity melatonin binding property, 18,19 most likely because of altered structural features in the 2nd extracellular loop (ECL2) important for melatonin binding.…”