Extrapolating from data in EGFR-mutated NSCLC, it is worth noting that for slow, asymptomatic progression, immediate change in therapy is not always required and patients may be able to continue a given ROS1 TKI while being cautiously monitored for change in the pace of PD or development of disease-related symptoms.64,65 In the face of significant systemic progression necessitating a switch in therapy, knowledge of the molecular mechanism of drug resistance (Figure 3) can inform subsequent treatment selection. The gene discussed is EGFR; the disease is non-small cell lung carcinoma.