ROS1 and neoplasm: Since the discovery of ROS1 gene fusions in non–small cell lung cancer (NSCLC) in 2007, our understanding of disease biology and therapeutic strategies has evolved remarkably, leading to improved patient outcomes.1  ROS1 gene fusions are established drivers across diverse types of adult and pediatric cancers2 and result in the expression of a chimeric oncoprotein in which the tyrosine kinase domain of ROS1 is fused to a non-native N-terminal binding partner.3 Aberrant expression and constitutive activation of ROS1 lead to unchecked proliferation of tumor cells.