We investigate the preclinical antitumor activity of DS-6051b using a variety of preclinical models, including: (1) Ba/F3 and other cell lines expressing the NTRK1 fusion or CD74-ROS1 fusion protein with or without resistant mutations, (2) ROS1-rearranged cancer patient-derived cell lines, and (3) in vivo models. The gene discussed is ROS1; the disease is cancer.