 Temsirolimus is a specific inhibitor of the mammalian target of rapamycin mTOR, an enzyme that regulates cell growth and proliferation. Temsirolimus prevents progression from the G1 phase to the S phase of the cell cycle through inhibition of mTOR, which is a novel mechanism of action for an anticancer drug. This is also important for concurrent treatment with radiation, since S-phase represents the most radiation resistant phase of the cell cycle. 