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Delay between initiation and promotion on mouse skin was in 1949 reported by Berenblum and Shubik not to affect tumour yields, and this led to the important concept of the irreversibility of initiation and stimulated the development of multistage models. Subsequent reports have, however, suggested that delay does decrease tumour yields, and this is confirmed by the present study of 2200 mice initiated at 8, 48, or 68 weeks with 10, 30, 100, or 300 microgram of DMBA and promoted by a standard dose of TPA for 15 weeks, after various delays. However, our data suggest that the decrease in tumour yields is chiefly or wholly due to a reduction, among ageing mice, of the ability to respond to promoters, and not to any substantial loss of initiated cells, for late initiation with immediate promotion also yielded a less rapid response than early initiation with immediate promotion. Interpretation of all such studies is complicated by the few weeks that the skin needs to repair ulceration and other damage induced by the higher doses of DMBA, for if promotion with TPA begins before such repair is complete the tumour yield may be misleadingly increased.

Initiation and promotion at different ages and doses in 2200 mice. I. Methods, and the apparent persistence of initiated cells