Glutamine is an important amino acid from a metabolic point of view. As an amino acid and a precursor for other amino acids, it sustains protein synthesis. It is also required for production of nucleotides (pyrimidines) and α-ketoglutarate (αKG). αKG is produced via double deamination of glutamine, a process termed glutaminolysis. Glutamine is first de-aminated by glutaminase (GLS) to produce glutamate. Glutamate is then converted to αKG by glutamate dehydrogenase (GDH). The production of αKG from glutamine is critical to replenish the tricarboxylic acid (TCA) cycle and thus to sustain ATP levels. The metabolic importance of glutamine is also reflected in the fact that cancer cells are often glutamine addicted. Intriguingly, the catabolism of glutamine by cancer cells exceeds the cellular requirement for glutamine in the production of amino acids, nucleotides and energy. What else is glutamine used for that could explain the glutamine addiction of cancer cells? Recent findings by Duran et al. demonstrate that glutaminolysis activates mammalian target of rapamycin (mTOR), hence inhibiting autophagy and promoting cell growth.1 This suggests that glutamine metabolism is additionally required as part of a signaling process to upregulate cell growth and proliferation.