“…This single treatment with a mutagen apparently causes cells to undergo a permanent change so that, for the rest of the mouse's life, its skin has become susceptible to the production of papillomas and carcinomas on exposure to the non-mutagenic irritants in croton oil. Surprisingly, a similar sequence has been seen using cells in vitro, where brief exposure to X-rays or methylcholanthrene apparently produces a permanent change in every cell so that, many generations later, its descendants occasionally undergo 'spontaneous' transformation into cancer cells (Kennedy et al, 1980(Kennedy et al, , 1984. Why does there have to be a long interval between a cell's exposure to a mutagen and the expression of the resulting mutations, and why do only a minority of the cell's descendants express these mutations?…”