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Main article: Warburg hypothesis
Warburg hypothesized that cancer growth is caused by tumor cells mainly generating energy (as e.g. adenosine triphosphate / ATP) by anaerobic breakdown of glucose (known as fermentation, or anaerobic respiration). This is in contrast to healthy cells, which mainly generate energy from oxidative breakdown ofpyruvate. Pyruvate is an end product of glycolysis, and is oxidized within the mitochondria. Hence, and according to Warburg, cancer should be interpreted as a mitochondrial dysfunction.
Cancer, above all other diseases, has countless secondary causes. But, even for cancer, there is only one prime cause. Summarized in a few words, the prime cause of cancer is the replacement of the respiration of oxygen in normal body cells by a fermentation of sugar.
— Otto H. Warburg, 
Warburg continued to develop the hypothesis experimentally, and held several prominent lectures outlining the theory and the data.
Today, mutations in oncogenes and tumor suppressor genes are thought to be responsible for malignant transformation, and the metabolic changes are considered to be a result of these mutations rather than a cause.