SUMMARY Rare and de novo single nucleotide variants (SNVs) and copy number variants (CNVs) are major risk factors for Neurodevelopmental Disorders (NDDs). The majority of NDD-associated SNVs affect a single allele of a gene, leading to haploinsufficiency. Correcting haploinsufficiency by increasing the expression level of the deficient allele could provide an attractive strategy for NDDs treatment. A variant of CRISPR, CRISPRa (CRISPR activation), offers the possibility to modulate the expression of endogenous genes by directly targeting their promoters or enhancers. Here, we are proposing to apply CRISPRa to upregulate the levels of Cul3 ubiquitin ligase, a high-confidence gene for NDDs. We have recently generated a haploinsufficient Cul3 mouse model. Brain MRI found decreased volume of cortical regions starting from early postnatal development and persisting into adulthood. Spatiotemporal transcriptomic and proteomic profiling implicated cytoskeletal and synaptic defects as key drivers of Cul3 functional impact. Specifically, dendritic growth, filamentous actin puncta, and spontaneous network activity measured by multielectrode arrays (MEA) were reduced in Cul3 mutant mice. Cul3 mutant mice also exhibited hyperactive behavior, along with social and cognitive deficits. We hypothesize that upregulation of Cul3 dosage early in development with CRSIPRa will rescue some (or all) of the observed phenotypes, and will lay the basis for general therapeutic interventions in NDDs. The goal of this project is to demonstrate the feasibility of compensating for Cul3 haploinsufficiency by rebalancing neuronal, molecular, cellular and network activity phenotypes. We will achieve this goal through the following Specific Aims: (1) To evaluate rescue potential of cellular and molecular phenotypes in the Cul3+/- CRISPRa mice; (2) To evaluate rescue potential of brain architecture and cognitive deficits in Cul3+/- CRISPRa mice. Our study will represent an effective strategy for rebalancing Cul3 (and potentially other NDD genes) deficiency and correcting associated phenotypes.