ABCG2 and neoplasm: RT-PCR analysis of Melanoma Associated Antigens (MAA) expressed by melanoma cell lines IGR37 and IGR39 sorted for its expression of CXCR6 and double positive ABCG2/CXCR6 IGR37 cells and in tumor xenograft engrafted with CXCR6+ IGR37 cells.