Cancer stem cells have been isolated from a number of human tumours. The latest example is a subpopulation of human malignant melanoma initiating cells, identified by their expression of the chemoresistance mediator ABCB5. The size of the ABCB5⁺ subpopulations correlates with clinical disease progression in patients with melanomas, and preliminary evidence also suggests that these melanoma stem cells can be specifically targeted with antibodies against ABCB5. This offers a potential therapeutic strategy against melanomas, and the study of cells of this type could help answer important questions in cancer biology. The hybrid melanoma cell shown on the cover, depicted as a merged, computer-enhanced fluorescent microscopy image, arose in vivo in a human tumour xenograft through fusion of an ABCB5⁺ melanoma stem cell with a more differentiated, ABCB5^- tumour cell. Nuclei are marked by genetically encoded red (DsRed) and green (EYFP) fluorescent labels, respectively.