Illustration of the British physician Peter Ratcliffe (born 1954). Ratcliffe is best known for his work on the mechanisms underlying cellular responses to low oxygen levels (hypoxia). While studying the production of erythropoietin (EPO), a hormone that stimulates the production of red blood cells in hypoxia, in kidney cells he realised that oxygen-sensing pathways were present in other organs. He also found that the pathways directed a range of other responses to hypoxia. Ratcliffe discovered the molecular mechanisms underlying the oxygen sensing and signalling pathways. He was awarded a share of the 2019 Nobel Prize in Physiology or Medicine along with Gregg L. Semenza and William G. Kaelin Jr. for this work.