Neuroepithelial cells are the progenitors for all the neurons in the mammalian central nervous system. At the onset of neurogenesis, the G1 phase of the cell cycle in neuroepithelial cells lengthens, but is this a cause or an effect of neurogenesis? To find out, Federico Calegari and Wieland Huttner treated 9.5-day mouse embryos in culture with olomoucine, an inhibitor of cyclin-dependent kinases that lengthens the G1 phase of the cell cycle (see p. 4947). In these cultures, TIS21, a marker for neuroepithelial cells that have switched from proliferative to neuron-generating divisions, was expressed prematurely and neurons were made earlier than expected. Because the only observable effect of olomoucine in these cultures was a lengthening of the cell cycle by about two hours, the researchers conclude that this change is sufficient to induce neuroepithelial cell differentiation. They therefore propose a model whereby cell-cycle length can be linked to the effects of cell-fate determinants.