MTOR and neoplasm: While other growth promoting pathways may be involved in tumor regrowth in mice treated with a single agent, it is possible that tumor proliferation is driven by a subtle shift in signaling towards the mTOR pathway in tumors treated with trametinib and towards the MAPK pathway in tumors treated with rapamycin, emphasizing the importance of simultaneous inhibition of both pathways in order to achieve a sustained response.