In this post hoc, interpatient comparative effectiveness study, patient cohorts from the LIBRETTO-001 trial were used to compare investigator-assessed outcomes associated with 1L selpercatinib versus 1L standard therapies in the frontline treatment of RET fusion-positive aNSCLC, RET-mutated MTC, and RET fusion-positive TC. This evidence concerns the gene RET and medullary thyroid gland carcinoma.