In patients with EGFR-mutant NSCLS, it was previously shown that an allele-specific assay was able to detect the emergence of T790M mutations in CTCs during first-line therapy.70 Subsequently, other studies confirmed that the analysis of lung CTCs can enable the monitoring of evolving tumour genotype in some patients.71–74 In addition to their genotype, physical characteristics of NSCLC CTCs have been studied, revealing that NSCLC CTCs appear as single or clustered, with the latter being mostly negative for the proliferation marker Ki67.75 This evidence concerns the gene EGFR and neoplasm.