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D. A protein which controls progression through the cell cycle.Tumour suppressor genes code for proteins which normally control progression through a cell cycle checkpoint. These genes only contribute to a cancer when both copies are mutated in a way which produces an inactive protein product. A single mutation inactivating one gene only has no effect. They contribute to cancer because the absence of the protein product removes controls on cell division. When genes coding for proteins forming part of a growth factor signalling pathway, or coding for proteins which help prevent apoptosis become mutated and contribute to cancers, they become oncogenes. They produce proteins which have gained a function. They contribute to cancers by stimulating cell growth, or by preventing a damaged cell from killing itself. Mutations in genes coding for DNA repair enzymes contribute to cancers in the same way that tumour suppressor genes do, because in their absence damaged DNA cannot be repaired, but they are not conventionally regarded as tumour suppressors.

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