Cell cycle dysregulation leads to abnormal proliferation and cell death in a context-specific manner. Cell cycle progression driven via the Rb pathway forces neurons to undergo S-phase, resulting in cell death associated with the progression of neuronal degeneration. Nevertheless, some Rb- and Rb family (Rb, p107 and p130)-deficient differentiating neurons can proliferate and form tumors. Here, we found in mouse that differentiating cerebral cortical excitatory neurons underwent S-phase progression but not cell division after acute Rb family inactivation in differentiating neurons. However, the differentiating neurons underwent cell division and proliferated when Rb family members were inactivated in cortical progenitors. Differentiating neurons generated from RbÐ(/)Ð; p107Ð(/)Ð; p130Ð(/)Ð (Rb-TKO) progenitors, but not acutely inactivated Rb-TKO differentiating neurons, activated the DNA double-strand break (DSB) repair pathway without increasing trimethylation at lysine 20 of histone H4 (H4K20), which has a role in protection against DNA damage. The activation of the DSB repair pathway was essential for the cell division of Rb-TKO differentiating neurons. These results suggest that newly born cortical neurons from progenitors become epigenetically protected from DNA damage and cell division in an Rb family-dependent manner.