CDK4 and neoplasm: A study [62] showed that the CDK4/6 inhibitor, palbociclib, strongly inhibited ATC proliferation in a xenograft ATC model but was susceptible to developing resistance; whereas combined treatment with PI3K/mTOR and CDK4/6 inhibitors synergistically reduced cell proliferation and was highly influential in inhibiting tumor growth even in cell lines that did not carry PI3K activating mutations or apply a low dose of the two drugs.