The proneural factor Ascl1 is involved in several steps of neurogenesis, from neural progenitor maintenance to initiation of terminal differentiation and neuronal subtype specification, integrating signals that drive neural stem cell maintenance or differentiation. Our study shows that that Ascl1a/b play only a minor or no role in the maintenance of their progenitor pools, but rather contribute to the initiation of terminal differentiation of GABAergic catecholaminergic neurons.