Otto Heinrich Warburg, son of physicist Emil Warburg, was a German physiologist, medical doctor and Nobel laureate. He was awarded the Nobel Prize in Physiology in 1931. Warburg investigated the metabolism of tumors and the respiration of cells, particularly cancer cells. In 1924, Warburg hypothesized that cancer, malignant growth, and tumor growth are caused by tumor cells mainly generating energy (as e.g. adenosine triphosphate/ATP) by nonoxidative breakdown of glucose (glycolysis) and the subsequent recycling of the metabolite NADH back to its oxidized form, for reuse in the glycolytic cycle to complete the process (known as fermentation, or anaerobic respiration). This is in contrast to “healthy” cells, which mainly generate energy from oxidative breakdown of pyruvate. Pyruvate is an end product of glycolysis, and is oxidized within the mitochondria. Therefore according to Warburg, cancer should be interpreted as a mitochondrial dysfunction.