Abstract: The invention provides the identification of the presence of mutant ROS protein in human cancer. In some embodiments, the mutant ROS are FIG-ROS fusion proteins comprising part of the FIG protein fused to the kinase domain of the ROS kinase. In some embodiments, the mutant ROS is the overexpression of wild-type ROS in cancerous tissues (or tissues suspected of being cancerous) where, in normal tissue of that same tissue type, ROS is not expressed or is expressed at lower levels. The mutant ROS proteins of the invention are anticipated to drive the proliferation and survival of a subgroup of human cancers, particularly in cancers of the liver (including bile duct), pancreas, kidney, and testes. The invention therefore provides, in part, isolated polynucleotides and vectors encoding the disclosed mutant ROS polypeptides (e.g., a FIG-ROS(S) fusion polypeptide), probes for detecting it, isolated mutant polypeptides, recombinant polypeptides, and reagents for detecting the fusion and truncated polypeptides.