Supplemental Results and Discussion Insm1 Expression Is Controlled by Positive and Negative Transcriptional Regulators of

The onset of neurogenesis in the various regions of the mammalian brain is controlled by a complex interplay of transcriptional regulators (Guillemot, 2005; Kageyama et al., 2005). Knockout mice in the Hes genes show a premature onset of neurogenesis (Guillemot, 2005; Kageyama et al., 2005). Remarkably, in the E10.5 hindbrain of Hes5 knockout mice, a brain region in which the phenotype with regard to neurogenesis is more profound than in the E10.5 dTel (due to lesser compensation by other Hes genes) (Hatakeyama et al., 2004; Ross et al., 2003), Insm1 expression in the VZ was markedly increased (Figure S2). This increase reflected the fact that in the absence of Hes5, a much greater proportion (≈50%) of APs than normal showed Insm1 expression at this early stage of neurogenesis (Fig. 2H, Figure S2). This increased Insm1 expression may be mediated by proneural genes, as these are known to be upregulated in the Hes5 knockout (Hatakeyama et al., 2004) and drive Insm1 expression ((Castro et al., 2006); and Figures S2 and 3 below). Next, we investigated whether the expression of Insm1 in neurogenic PCs was dependent on proneural genes, which are known to induce NE PCs to become neurogenic (Gohlke et al., 2008; Guillemot, 2005). Indeed, in E10.0 Mash1 and E10.5 Ngn2 knockout mice, in which th