Experiments performed with HIF1α-/- embryonic stem (ES) cells conflictingly revealed that HIF1 may act as a positive regulator of tumour growth, most likely through its activation of vascular endothelial growth factor (VEGF) expression [3] or as a negative regulator, possibly through the stabilization of p53 in hypoxic cells [4]. The gene discussed is VEGFA; the disease is neoplasm.