In work done in the 1990s using genetically engineered mice, Gregg Semenza, the director of the vascular program at John Hopkins University’s Institute for Cell Engineering, located segments of DNA next to the EPO gene. This gene codes for the hormone erythropoietin. These DNA segments mediated the response to hypoxia. During the same time period, another researcher, Peter Ratcliffe who heads up clinical research at the Francis Crick Institute in London, also identified how oxygen regulated the EPO gene. Both research groups found that the oxygen-sensing mechanism was present in virtually all tissues—not just in the kidney cells, where EPO is produced. This finding revealed that the mechanism was functional in many different types of cells.