Cellular response to oxygen depletion is mediated by HIF (hypoxia-inducible factor). HIF is a heterodimer consisting of a constitutively expressed subunit (HIFbeta) and an oxygen-regulated subunit (HIFalpha). HIFalpha stability is regulated by prolyl hydroxylation by PHD (prolyl hydroxylase domain-containing protein) family members. PHD activity depends on the availability of molecular oxygen, making PHDs the oxygen-sensing system in animal cells. However, PHDs have recently been shown to respond to stimuli other than oxygen, such as 2-oxoglutarate (alpha-ketoglutarate), succinate or fumarate, as illustrated by the pseudo-hypoxic response in succinate dehydrogenase- or fumarate dehydrogenase-deficient tumours. Moreover, HIFalpha is not the sole PHD effector, suggesting that PHDs have functions that extend beyond oxygen sensing. Currently, we are investigating the role of PHDs in the cellular response to amino acid deprivation, a process regulated by mTOR (mammalian target of rapamycin). The precise mechanism whereby amino acids are signalling to mTOR is not fully understood. Given that 2-oxoglutarate is a limiting co-substrate for PHD activity during normoxia and that 2-oxoglutarate levels depend on amino acid availability, it is possible that PHD activity depends not only on oxygen, but also on amino acid availability, suggesting a global metabolic sensor function for PHDs which could be signalling not only to HIF, but also to mTOR.