question: What is known about mammalian melatonin receptors?
answer: Melatonin receptors MT1 and MT2 (genes officially named MTNR1A and MTNR1B, respectively) play crucial roles in melatonin-mediated regulation of circadian rhythms, the immune system, and control of reproduction in seasonally breeding animals.
  The melatonin receptor family is a small group of receptors within the G protein-coupled receptor (GPCR) superfamily. The group comprises of three subtypes which bind melatonin and one member, the melatonin related receptor (MRR), that shares >40% sequence identity with the other melatonin receptors but does not bind melatonin.