A proof-of-principle study in lung cancer detected activating mutations in the gene encoding the epidermal growth factor receptor (EGFR) in CTCs from 11 out of the 12 patients tested; these patients were receiving EGFR-targeting tyrosine kinase inhibitors, and interestingly some of these mutations in CTCs had emerged de novo (that is, were not present in the matched primary tumor), suggestive of temporal evolution and a putative acquired mechanism of resistance to therapy [48]. The gene discussed is EGFR; the disease is lung cancer.