||"During its entire lifetime, however, the rat produces and discards about 10^13 epithelial cells from its small intestine alone, and almost as many cells from various other epithelia (Enesco & Leblond, Increase in Cell Number as a Factor in the Growth of the Organs and Tissues of the Young Male Rat, Embryol. exp. morph. 10, 530-562 (1962)). Its risk of being overwhelmed by variants, is, in principle at least, related to this total of 10^13. For man, who lives ten times as long, and is 100 times as large, there must be some 10^16 cell divisions, and this number must somehow be reconciled with a spontaneous mutation rate of about 10^-6 per gene per cell division (refs 2-4). Because most of the cell division is occurring in epithelia, that is where [researchers] may expect to find the protective mechanisms most highly developed."