Figure 1. The initiation/promotion/progression model of carcinogenesis. 
1=rate of terminal differentiation and death of stem cell daughter; 2=blockage of normal terminal differentiation of the initiated stem cell (---||–
) but not of the proliferation of the initiated cell. This initiated cell can no longer divide asymmetrically but can divide symmetrically; 
1=rate of stem cell renewal after stem cell divides asymmetrically; 2=rate of cell division of initiated cells; µ1=rate of the molecular event leading to initiation (i.e. mutation); µ2=rate at which the second event occurs within an initiated cell. (With permission from: Trosko et al. In: Hertberg EL, Johnson RG, editors. Gap junctions: Modern cell biology, Vol. 7. New York, NY: Alan R Liss Inc., 1998:435–48.)