Professor Peter Ratcliffe is Professor of Renal Medicine in the Nuffield Department of Medicine at the University of Oxford. His work has focused on the oxygen sensing processes in the body. Through an outstanding series of experimental studies he has characterised hypoxia-inducible factor as a transcriptional complex that plays a central role in the regulation of genes responsive to oxygen concentration. He has shown that a post-translational modification of HIF by probyl hydroxylation is a key regulatory event in the control of HIF activity. Most recently he has identified the enzyme responsible for this post-translational modification as member of a family of dioxygenase enzymes that is sensitive to graded hypoxia.