DLG4-related synaptopathy is a newly identified neurological disorder associated with de novo heterozygous pathogenic variants in the Discs Large Homolog 4 (DLG4) gene, which encodes the postsynaptic density protein PSD-95. Clinical manifestations are heterogeneous and include intellectual disability, developmental delay, epilepsy, autism spectrum disorder, schizophrenia, language, and cognitive impairments. Given the central role of PSD-95 in multiple synaptic signaling pathways, patient-identified variants give rise to phenotypes consistent with disruption of its broader interactome. Unfortunately, no genotype–phenotype correlations have been established, and treatment is limited to symptom management through supportive care and neurological monitoring. We employ patient-derived induced pluripotent stem cell (iPSC) models to investigate disease mechanisms in a variant-specific manner, focusing on three representative DLG4 variants (missense, nonsense, and frameshift). Using 2D neuronal cultures, we identified haploinsufficient synaptic deficits that can be rescued by AAV9-hSyn-DLG4 gene delivery. Building on these findings, we propose extending our studies to 3D brain organoids to examine neurodevelopmental defects and altered crosstalk between cellular and extracellular signaling in a more physiologically relevant context. We will further evaluate two IP-protected adeno-associated virus serotype 9 (AAV9) vectors to extend the relevance of our findings to other neurodevelopmental and psychiatric disorders with altered PSD-95 signaling, including schizophrenia and autism.