ROS1 and non-small cell lung carcinoma: Although a rare occurrence in ROS1+ NSCLC, small cell transformation has been described in approximately 2% of patients with ROS1 TKI-resistant disease, with inactivation of RB1 and TP53 and loss of ROS1 fusion RNA and protein expression.73 This is similar to small cell transformation in EGFR-mutated NSCLC, where dependence on EGFR is lost.