ROS1 is a  tyrosine kinase receptor (RTK) of the insulin receptor family. Chromosomal rearrangements (fusions) involving the ROS1 gene, on chromosome 6q22, result in a downstream  activation of the  cellular pathways known to be involved in cell growth and in cell proliferation.

Approximately 2% of lung tumors harbor ROS1 fusions. As well as for ALK fusions, ROS1 fusions are more commonly found in light smokers (<10 pack years) and/or never-smokers. ROS1 fusions are also associated with younger age and adenocarcinomas.

ROS1 fusions are associated with sensitivity in vitro to tyrosine kinase inhibitors which inhibit ROS1, such as crizotinib. Therefore it is an important molecular target to search for the therapeutic possibilities that implies.