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Abstract
After years of observing children with the rare eye cancer retinoblastoma, Alfred Knudson proposed an explanation for how two different forms of it arise. His 'two-hit' hypothesis led to the realization that the loss of gene function, not just the activation of a cancer-causing gene, could cause cancer. Knudson, who was born in Los Angeles, California, in 1922, died on 10 July, aged 93.