ROS1, an orphan receptor tyrosine kinase of the insulin receptor family, was initially identified as a homolog of v-Ros from the UR2 sarcoma virus. ROS1 consists of a large extracellular domain composed of nine fibronectin repeats, a transmembrane domain, and an intracellular kinase domain. While the function of ROS1 is undefined, it has been shown to play an essential role in the differentiation of epididymal epithelium.