Molecular results guide second-line, molecularly stratified treatment, including FGFR inhibition for FGFR2 fusions/rearrangements, IDH1 inhibition for IDH1-mutant tumors, HER2-directed therapy for ERBB2-amplified disease, immune checkpoint inhibition for MSI-H/MMRd tumors, and TRK inhibition for NTRK fusion–positive cancers. This evidence concerns the gene IDH1 and cancer.