CD44 and Merkel cell skin cancer: To select markers that could be used to identify circulating MCC cells in blood samples, first we determined the phenotype of three MCC cell lines (MCCL-9, MCCL-11 and MKL-1) using markers that are commonly employed for the histopathological diagnosis of this cancer (Neuron-Specific Enolase (NSE); Synaptophysin, Chromogranin A; Cytokeratin 20 (CK20); and CD56) and markers usually used for CTCs detection (Epithelial Cell Adhesion Molecule (EpCAM); PanCK (8, 18, 19); Vimentin; CD24, CD44, CD45) (Fig. 1 and Supplementary Fig. S1).