The mammalian target of rapamycin (mTOR), a 290 kDa serine/threonine kinase, is crucial for protein synthesis, cell growth, proliferation, and differentiation. It's inhibited by rapamycin, an immunosuppressant. mTOR's activity is tightly controlled by growth factors and nutrients such as glucose and amino acids. It's involved in various pathways, from protein degradation via autophagy to protein synthesis via translation initiation (4EBP), ribosome, and lipid synthesis. It's found in vesicles and the cytosol.