Building a functional central nervous system involves tight regulation between proliferation of neural stem/progenitor cells and their differentiation in neurons. A deregulation of this balance can lead to neurodevelopmental disorders, such as microcephaly, when the pool of progenitor is depleted due to precocious neuronal differentiation, or tumors growth if there is an excess of proliferation. Accumulating data underline the role of cell cycle regulator in this process. Using RNAi and gain of function approaches in the chick embryo, our team recently showed that the CDC25B phosphatase, a positive cell cycle regulator, promotes neuron production in the developing spinal cord (Peco et al., 2012; Bonnet et al., in preparation). We now developed a mouse genetic tool in order to asses CDC25B functional conservation in mammals neurogenesis and in a more complex organ such as the telencephalon.