ABCG2 and melanoma: The existence of melanoma-initiating cells has only been recently proposed, with the description of a subpopulation of melanoma cells expressing markers such as CD20 [258], CD133 [259], CD24 [260], CD271 [261], or the ABC transporter (ATP-binding cassette) involved in drug efflux (ABCB5) [262], ALDH1A [263].