ROS1 is a type I integral membrane protein with tyrosine kinase activity of the insulin receptor family. ROS1 consists of a large extracellular domain that is composed of six fibronectin repeats, a transmembrane domain, and a C-terminal kinase domain. Being an orphan receptor, the functions of ROS1 are not well known, though it has been shown to play an important role in differentiation of epididymal epithelium. This proto-oncogene, highly-expressed in a variety of tumor cell lines, belongs to the sevenless subfamily of tyrosine kinase insulin receptor genes. The first oncogenic fusion of ROS1, FIG-ROS1, was initially identified by research studies in glioblastoma, and subsequent studies have found this fusion in cholangiocarcinoma, ovarian cancer, and non-small cell lung cancer (NSCLC).