The mammalian target of rapamycin (mTOR), a member of the phosphoinositide 3-kinase related kinase (PIKK) family, plays a central role in the regulation of cell growth. The cellular function of mTOR has been proposed based solely on loss-of-function analyses using the specific inhibitor rapamycin or RNAi-mediated knockdown. There have been recent reports of mTOR mutants with enhanced activity that were isolated by genetic screening in yeast. These isolated mTOR mutants exhibited enhanced kinase activity in vitro, and when expressed in cells, prevented the dephosphorylation of known mTOR substrates. The application of these mutants in gain-of-function analyses has enabled a re-evaluation of the function of mTOR. Although these studies confirmed many of the proposed mTOR functions some unexpected observations urged a reconsideration of the regulatory mechanisms and the physiological function of the mTOR pathway. Hyperactive mTOR mutants are thus valuable tools for analysis of the activation mechanism as well as the in vivo function of mTOR.