TK1 and melanoma: Compared with the C3 subpopulation which is far from the malignant cells, the C5 subpopulation highly expresses a large number of genes related to the dissemination and metastasis of melanoma (Fig. 5e and Supplementary Fig. 26), including thymidine kinase 1 (TK1)55, NME/NM23 nucleoside diphosphate kinase 1 (NME1)56, and immunoglobulin superfamily member 8 (IGSF8)57.