Mice were treated according to institutional and national guidelines. α-Cre mice (P. Gruss), Chx10-Cre mice (C. Cepko), RbloxP/loxP mice (A. Berns), E2f1–/– mice, E2f2–/– mice, E2f3loxP/loxP mice, and E2f3a−/− mice were maintained on a mixed (NMRI × C57/Bl × FVB/N × 129sv) background. A detailed description of E2f3a−/− mice will be published elsewhere. Mice of different genotypes were compared within the same litter and across a minimum of three litters. We have not noted any phenotypic differences in separate litters. Genotyping was performed as before [2,5], and the primers used for genotyping E2f3a−/− mice were E2f3a KL (5′-CTCCAGACCCCCGATTATTT-3′), E2f3a KR1 (5′-TCCAGTGCACTACTCCCTCC-3′), and E2f3a KM (5′-GCTAGCAGTGCCCTTTTGTC-3′).