ROS1 and skin neoplasm: In the 4th edition of the 2018 WHO classification of skin tumors [13], lesions in the spitzoid pathway (pathway 4) are characterized by mutations in HRAS, tyrosine kinase fusions (ALK, ROS1, RET, NTRK1/3, and MET), or serine-threonine kinase fusions (BRAF, MAP3K8).