MTOR and osteosarcoma: When tested in the US National Cancer Institute–supported Pediatric Preclinical Testing Program, rapamycin, the prototypic mTOR inhibitor, induced significant differences in event-free survival distributions compared with controls in 27 of 36 in vivo solid tumor models, and objective responses were observed in select osteosarcoma, rhabdomyosarcoma, and rhabdoid tumor xenografts [8].