A great many aspects of the anatomy and physiology of large animals are constrained by the need to match oxygen supply to cellular metabolism and appear likely to involve the regulation of gene expression by oxygen. Some insight into possible underlying mechanisms has been provided by studies of erythropoietin, a haemopoietic growth factor which stimulates red cell production in response to hypoxia. Studies of hypoxia-inducible cis-acting sequences from the erythropoietin gene have led to the...