This study describes the detection of EGFR mutations in circulating tumor cells (CTCs) from patients with non–small-cell lung cancer (NSCLC). The researchers used a microfluidic device to isolate highly purified CTCs from blood samples and performed EGFR mutational analysis on DNA extracted from these cells. They compared the results with those from free plasma DNA and original tumor biopsy specimens. The study found that EGFR activating mutations were detected in 92% of patients with NSCLC and in 33% of patients with matched free plasma DNA. The T790M mutation, which confers drug resistance, was detected in CTCs from patients with EGFR mutations who had received tyrosine kinase inhibitors. The presence of T790M in pretreatment tumor biopsy specimens was associated with reduced progression-free survival. Serial analysis of CTCs showed that a reduction in the number of captured cells was associated with a radiographic tumor response, while an increase was associated with tumor progression and the emergence of additional EGFR mutations. The study concludes that molecular analysis of CTCs from the blood of patients with lung cancer offers the possibility of monitoring changes in epithelial tumor genotypes during treatment. The study also highlights the importance of CTC analysis in identifying drug resistance mutations and monitoring treatment response. The results suggest that CTC analysis could be a valuable tool for non-invasive monitoring of tumor genotypes in patients with NSCLC.This study describes the detection of EGFR mutations in circulating tumor cells (CTCs) from patients with non–small-cell lung cancer (NSCLC). The researchers used a microfluidic device to isolate highly purified CTCs from blood samples and performed EGFR mutational analysis on DNA extracted from these cells. They compared the results with those from free plasma DNA and original tumor biopsy specimens. The study found that EGFR activating mutations were detected in 92% of patients with NSCLC and in 33% of patients with matched free plasma DNA. The T790M mutation, which confers drug resistance, was detected in CTCs from patients with EGFR mutations who had received tyrosine kinase inhibitors. The presence of T790M in pretreatment tumor biopsy specimens was associated with reduced progression-free survival. Serial analysis of CTCs showed that a reduction in the number of captured cells was associated with a radiographic tumor response, while an increase was associated with tumor progression and the emergence of additional EGFR mutations. The study concludes that molecular analysis of CTCs from the blood of patients with lung cancer offers the possibility of monitoring changes in epithelial tumor genotypes during treatment. The study also highlights the importance of CTC analysis in identifying drug resistance mutations and monitoring treatment response. The results suggest that CTC analysis could be a valuable tool for non-invasive monitoring of tumor genotypes in patients with NSCLC.