The mammalian Target of Rapamycin (TOR, also known as mTOR) is an evolutionarily conserved serine/threonine kinase that regulates cell growth and cell cycle through its ability to integrate signals from nutrient levels and growth factors. It was initially discovered as a kinase whose ability to stimulate T cell proliferation in response to IL-2 could be inhibited by the immunosuppressive drug rapamycin. Rapamycin inhibits TOR in other cell types resulting in reduced cell growth and reduced rates of cell cycle and cell proliferation. TOR is normally associated with the regulatory proteins RAPTOR and GbL. Its downstream targets are thought to be the ribosomal protein S6 kinases and the eukaryotic initiation factor 4E binding proteins (4EBPs). Regulation of these protein families allows TOR to control protein biosynthesis.