The ROS1 gene encodes ROS proto-oncogene 1, a receptor tyrosine kinase. The ROS1 protein is an enzyme with a mass of 263.9 kDa. ROS1 gene rearrangements have been implicated in multiple cancer indications including non-small cell lung cancer (NSCLC), gastric cancer, glioblastoma multiforme, ovarian cancer, and colorectal cancer (1). ROS1 protein fusion expression leads to oncogenic transformation, most likely through constitutive activation of the tyrosine kinase domain (2). ROS1 gene rearrangements occur in 1-2% of NSCLC, and clinical trial data indicates that tumors positive for ROS1 protein fusions are sensitive to kinase inhibitors such as crizotinib, and entrectinib (3,4). For detection of positive ROS1 fusions, IHC testing has been shown 100% sensitivity and 92-94% specificity in recent studies, proving its efficacy as a screening tool for ROS1-targeted therapies (5,6).