In accordance with the invention, a novel gene translocation, (5q32, 6q22), in human non-small cell lung carcinoma (NSCLC) that results in a fusion proteins combining part of CD74 with Proto-oncogene Tyrosine Protein Kinase ROS Precursor (ROS) kinase has now been identified. The CD74-ROS fusion protein is anticipated to drive the proliferation and survival of a subgroup of NSCLC tumors. The invention therefore provides, in part, isolated polynucleotides and vectors encoding the disclosed mutant ROS kinase polypeptides, probes for detecting it, isolated mutant polypeptides, recombinant polypeptides, and reagents for detecting the fusion and truncated polypeptides. The disclosed identification of the new fusion protein enables new methods for determining the presence of these mutant ROS kinase polypeptides in a biological sample, methods for screening for compounds that inhibit the proteins, and methods for inhibiting the progression of a cancer characterized by the mutant polynucleotides or polypeptides, which are also provided by the invention.