Lower oxygen in the liver and kidney causes secretion of HIF-1α and HIF-2α, which tell cells to produce EPO — an essential component in erythrocyte production. Gene doping (refer to fig. 11) can be used to modify cells, ensuring continuous EPO production.

Alternatively, rEPO can be injected. However small changes in the structure can lengthen the amount of time it stays in the body, or decrease potential immune responses.

Measuring the ratio of EPO to reticulocytes, and how the response to EPO translates into more oxygen passing through the body via increases in haemoglobin, are both important considerations in choosing an athlete. EPO production and corresponding increases in erythrocyte production can vary hugely from person to person.