Tumours were induced by chemical carcinogens in chimaeric mice made by the amalgamation of two embryos each producing a different electrophoretic variant of the enzyme glucose phosphate isomerase. Although, in these chimaeras, the smallest samples of normal tissue that could be analysed almost invariably contained both isoenzymes, almost all the tumours contained only one. An analysis of the size of the clones that formed the chimaeric epidermis permitted the conclusion that the epidermal tumours produced could not have risen from more than 8 cells; and the most probable interpretation of the data is that these tumours were clonal growths.

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