PROJECT SUMMARY Schizophrenia, bipolar disorder and autism are common and debilitating neurodevelopmental disorders that together affect more than 5 million Americans. Despite more than fifty years of research, no cures exist and the standard of treatment remains unsatisfactory. Heterozygous mutations of neurexin-1 (NRXN1) have been repeatedly associated with schizophrenia (SZ) and autism spectrum disorder (ASD). The clinical presentations of NRXN1+/- mutations (including diagnosis, severity, prognosis and age-of-onset) in affected patients are diverse and the genetic mechanism affecting the penetrance of these mutations remains unknown. Moreover, mouse models do not permit researchers to study how and why some NRXN1+/- deletions have more deleterious effects in patients. Our objective is to resolve how NRXN1+/- deletions perturb the NRXN1 isoform repertoire and impact neuronal maturation and synaptic function. Our preliminary data defined the NRXN1 alternative splice repertoire in control fetal and adult cortical tissue, and compared this to human induced pluripotent stem cell (hiPSC)-derived neurons from NRXN1+/- cases and controls. Here, we propose to evaluate the effect of experimental manipulation of the NRXN1 isoform repertoire in both control and NRXN1+/- patient-derived excitatory and inhibitory neurons. Ultimately, we hope to directly correlate genomic and functional deficits across increasingly refined populations of NRXN1+/- patient-derived neurons.