Visualization of circulating tumor cells (CTCs) after isolation with the CellSearch semiautomated system (Janssen Diagnostics). (a) Two CTCs from a blood sample from a cancer patient. Both cells have an oval morphology and have a diameter >4 μm. The nucleus is visible with DAPI (DAP) and they are positively stained for epithelial markers (cytokeratins, CK) but not for CD45 (a leukocyte marker). (b) Two ‘events’ identified as CTCs by the automated system but that were discarded by the trained human operator as each does not fulfill the characteristics of a CTC. APC, allophycocyanin; PE, phycoerythrin.