ABCG2 and neoplasm: These cells are able to form melanospheres when cultured in appropriate conditions, express the embryonic SOX2 and KLF4 stem cell markers, are associated with a high invasive capacity, are more efficient than adherent cells in terms of engraftment and tumor formation when inoculated into nude mice, and are characterized by intracellular autofluorescence associated with ABCG2 expression.