Developmental analysis of Ki67 labeling. A Ki67 labeling index was determined in mice of different ages by multiplying the mean number of Ki67/insulin double-positive cells per section by β cell mass (μg). The values at E18.5 were normalized to 100%. None of the differences observed between the different genotypes are statistically significant. Two mice for each genotype were used at each time point, except for Igf1r^lox/+ and Igf1r^Δlox/+ at P10, where one mouse was used. Three to five sections per pancreas were scored as described in Methods.