Many cellular responses to changes in O2 availability are mediated through the hypoxia inducible transcription factor, HIF. HIF regulation is largely dependent on the activity of O2-dependent prolyl hydroxylases. Under normoxic conditions, these enzymes modify HIF to promote its proteasomal degradation. Recently we proposed a second function for the HIF prolyl hydroxylases--recruitment of the candidate tumor suppressor protein ING4 to HIF under hypoxic conditions. Rather than affecting hydroxylase activity or HIF stability, ING4 mediates the ability of HIF to activate transcription of its downstream target genes. This additional mechanism of HIF regulation may serve to fine-tune the magnitude of the cellular hypoxic response under physiological conditions. Furthermore, the identification of a link between ING4 and HIF may shed light on a mechanism by which misregulation of ING4 in tumors promotes angiogenesis, loss of contact inhibition, and changes in cellular proliferation and survival.