The mature cerebellum controls motor skill precision and participates in other sophisticated brain functions that include learning, cognition, and speech. Different types of GABAergic and glutamatergic cerebellar neurons originate in temporal order from two progenitor niches, the ventricular zone and rhombic lip, which express the transcription factors Ptf1a and Atoh1, respectively. However, the molecular machinery required to specify the distinct neuronal types emanating from these progenitor zones is still unclear. Here, we uncover the transcription factor Olig3 as a major determinant in generating the earliest neuronal derivatives emanating from both progenitor zones in mice. In the rhombic lip, Olig3 regulates progenitor cell proliferation. In the ventricular zone, Olig3 safeguards Purkinje cell specification by curtailing the expression of Pax2, a transcription factor that suppresses the Purkinje cell differentiation program. Our work thus defines Olig3 as a key factor in early cerebellar development.

The mature cerebellum controls motor skill precision and participates in other sophisticated brain functions that include learning, cognition and speech. Different types of GABAergic and glutamatergic cerebellar neurons originate in temporal order from two progenitor niches, the ventricular zone and rhombic lip, which express the transcription factors Ptf1a and Atoh1, respectively. However, the molecular machinery required to specify the distinct neuronal types emanating from these progenitor zones is still unclear. Here, we uncover the transcription factor Olig3 as a major determinant in generating the earliest neuronal derivatives emanating from both progenitor zones. In the rhombic lip, Olig3 regulates progenitor cell proliferation. In the ventricular zone, Olig3 safeguards Purkinje cell specification by curtailing the expression of Pax2, a transcription factor that we found to impose an inhibitory interneuron identity. Our work thus defines Olig3 as a key factor in cerebellar development. ### Competing Interest Statement The authors have declared no competing interest.

The aim of the study is to identify new gene mutations causing cerebral cortex malformations in mice. Materials and Methods. To identify genes causing cerebral cortex malformations, chemical mutagenesis was carried out using N-ethyl-N-nitrosourea as a mutagen. A total of 141 male C3H mice aged 8 weeks were injected with the mutagen in order to induce mutations in spermatogonial stem cells. After a period of sterility, the animals were used in three-generation backcross scheme. Satb2-LacZ reporter mice were involved in this strategy to label the neurons forming the corpus callosum. Results. The animal phenotype displaying primary microcephaly and 6 mutant lines demonstrating audiogenic epilepsy have been described. The phenotypes of these mutants will be further presented and discussed.