The invention provides the identification of the presence of polypeptides with ROS kinase activity in mammalian lung cancer. In some embodiments, the polypeptide with ROS kinase activity is the result of a fusion between a ROS-encoding polynucleotide and a polynucleotide encoding a second (non-ROS) polypeptide. Three different fusion partners of ROS are described, namely proteins encoded by the FIG gene, the SLC34A2 gene, and the CD74 gene. The invention enables new methods for determining the presence of a polypeptide with ROS kinase activity in a biological sample, methods for screening for compounds that inhibit the proteins, and methods for inhibiting the progression of a cancer (e.g., an lung cancer).