Complex sentence: The MT 1 (or Mel 1a or MTNR1A) and MT 2 (or Mel 1b or MTNR1B) receptor subtypes are present in humans and other mammals, while an additional melatonin receptor subtype Mel 1c has been identified in amphibia and birds.
Split into simpler sentences: The MT 1 (or Mel 1a or MTNR1A) and MT 2 (or Mel 1b or MTNR1B) receptor subtypes are present in humans and other mammals. While an additional melatonin receptor subtype Mel 1c has been identified in amphibia and birds.