Extended Data Figure 1.

A. Expression of early forebrain marker genes of PAX6, HES5, EOMES (TBR2) and SYNGAP1 from post-conception day (PCD) 26 to 54 from single cell RNA-seq data.

B. UMAP visualization of age-dependent clustering of fetal single cells.

C. SYNGAP1 expression at PCD 56 grouped by cell types; intermediate progenitor cells (IPC), neuroepithelial cells (NE), radial glial cells (RGCs) and neurons.

D. D.I.V. 7 cortical organoids are composed of cells positive for the neural stem cell marker SOX2, the radial glial progenitor marker PAX6, the nuclear marker DAPI and SYNGAP1.

E. A coronal section from E13.5 mouse brain showing expression of the neural stem cell marker SOX2, the tight junction protein TJP1, and SYNGAP1. SYNGAP1 is highly expressed at the ventricular wall. White box indicates the Region of Interest selected for the merged images showing colocalization of DAPI, TJP1, and SYNGAP1.

F. Peptide competition assay shows the specificity of the SYNGAP1 antibody used. 5X and 10X concentrations of the commercial antigenic peptide were evaluated, showing a strong reduction in specific signal in the apical wall of the ventricular zone.

G. SynGAP1 expression in E18.5 wild type and SynGAP1 KO mouse showing the overall decrease in SynGAP1 levels. Decreased levels of SynGAP are most evident at the VZ.