This flow cytometry assay identifies subpopulations of CTCs based on their positivity for melanoma-associated markers, MCAM (melanoma cell adhesion molecule) and MCSP (melanoma-associated chondroitin sulphate proteoglycan), and/or their positivity for melanoma-initiating cell markers, ABCB5 (ATP-binding cassette subfamily B member 5), CD271 and RANK (Receptor Activator of NFkβ pathway) [10]. The gene discussed is ABCB5; the disease is melanoma.