ROS1 and small cell lung carcinoma: Given the advances in biomarker testing in non-small cell carcinoma (e.g., epidermal growth factor receptor [EGFR], anaplastic lymphoma kinase [ALK], programmed death ligand-1 [PD-L1], ROS-1), testing should be done if a non-small carcinoma component cannot be excluded​​​​. Small cell carcinomas can evolve de novo or from non-small cell carcinoma; the clinical history and prior pathology may provide important clues that a case has evolved from non-small cell carcinoma and should get tested for drug-able molecular mutations/non-SCLC biomarkers.