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Tumorigenesis is initiated when the first ‘hit’, in the form of the inherited, mutant allelic gene, is succeeded by a second ‘hit’, a somatic mutation at the same site. The ‘retinoblastoma gene’, RB1 is located on the long arm of somatic chromosome 13, at 13q14. It has proved of importance in understanding the regulation of cell growth and carcinogenesis (p. 286). It is a universal ‘brake’.