The term 'nutrient sensing' has been defined and interpreted in different ways. In the narrowest sense, a nutrient sensor is a protein whose binding to a nutrient leads to a change inthe function of that protein, which in some cases may triggerdownstream activation of a signalling cascade. Nutrients suchas AAs (amino acids), glucose and fatty acids have traditionallybeen considered as metabolic fuels for cell growthand development. But this view is undergoing a rapid changewith an increased understanding of the mechanisms bywhich such nutrients can act as mediators of signal transductionpathways. These nutrient-mediated signalling cascades regulate specific aspects of fuel and energy metabolism,ultimately influencing cell growth, proliferation and survival.