Figure 8.

. Summary of the phenotypic analysis of Ngn2^KIMash1 and Mash1^KINgn2mice. The different regions of the CNS and PNS analyzed in the two KI mouse strains are schematized. Neurogenesis defects observed in null mutant mice are rescued in KI mice (indicated by RN +), except in tissues that are missing due to specification defects (indicated by RN nd). The rescue of specification defects (indicated by RSS +) varies from lineage to lineage in both KI strains, reflecting the variable contribution of proneural genes to the specification of neuronal subtype identity in different lineages. Respecification of neuronal subtypes caused by ectopic proneural gene expression (indicated by RNS +) is observed only in the CNS of Ngn2^KIMash1 mice, indicating that Mash1 but not Ngn2 has an instructive role in the specification of neuronal identity in the CNS.