Genotyping of single CTCs isolated by the CellSearch or the IsoFlux system in patients with mCRC confirmed an intra- and inter- patient heterogeneity based on the PIK3CA and KRAS mutation status [22], [31]; moreover, different genetic alterations on single CTCs have already been reported in patients with breast and esophageal cancer [23], [40]. The gene discussed is KRAS; the disease is esophageal cancer.