Numerous melanocortin, MCH and melatonin receptors have been identified in fish and frogs, including a homologue of "MC1R", a melanocortin receptor known to regulate skin and hair colour in humans. It has been demonstrated that "MC1R" is required in zebrafish for dispersion of melanin. Inside the cell, cyclic adenosine monophosphate (cAMP) has been shown to be an important second messenger of pigment translocation. Through a mechanism not yet fully understood, cAMP influences other proteins such as protein kinase A to drive molecular motors carrying pigment containing vesicles along both microtubules and microfilaments.