As a mTOR inhibitor, temsirolimus (cell cycle inhibitor-779, CCI 779) can inhibit mTOR with IC50 of 1.76 uM. Temsirolimus (cell cycle inhibitor-779, CCI 779) is an ester analogue of sirolimus for the treatment of cancer, multiple sclerosis and rheumatoid arthritis. Temsirolimus binds to the cytosolic protein, FKBP, which subsequently inhibits mTOR (mammalian target of rapamycin). Inhibition of mTOR blocks a number of signal transduction pathways that suppress translation of several key proteins regulating the cell cycle. By inhibiting mTOR, temsirolimus lead to a cell cycle block at the G1 phase.