Test Details: ROS1 rearrangements testing is usually done for people who are negative for KRAS, EGFR mutations and ALK gene rearrangements. ROS1 gene rearrangements are resistant to epidermal growth factor receptor tyrosine kinase inhibitors, but may be highly sensitive to ROS1 inhibitors, like Xalkori (crizotinib). The drug Xalkori works by blocking certain kinases, including those produced by the abnormal ROS1 gene. Crizotinib treatment of patients with tumours exhibiting ROS1 rearrangements can halt tumour progression or result in tumour regression.