Erythropoietin, a hormone produced in the adult kidney, controls erythrocyte production. In response to ischemic or hypoxic hypoxia, the level of expression of the gene increases markedly. We have a limited understanding of the factors governing the tissue specificity of erythropoietin gene expression but considerable progress has been made toward understanding the mechanisms that increase erythropoietin gene expression in response to hypoxia. An increase in transcription occurs, mediated (at least in part) by formation of a heterodimeric DNA binding complex termed hypoxia inducible factor 1. Transcriptional regulation mediated by this complex has now been shown to modulate the expression of a number of other genes in a wide range of cell types.