The core concept of our proposed studies is that sex-biased gene expression early in human development pattern the brain differently in males and females, and that this patterning underlies differential susceptibility of males and females to neurodevelopmental disorders. We envision that these sex differences are regulated by epigenetic programs controlling the expression of developmentally regulated genes in a sex-biased manner. We further postulate that these sex- dependent epigenetic mechanisms interact with genetic risk factors that influence risk, progression, and severity of autism sprectrum disorders (ASD) and schizophrenia (SCZ), and we will test these hypotheses in two specific aims. Aim 1 is to delineate the sex-dependent regulation of early brain development in typical individuals. Epigenetic and gene expression programs in brain organoids and postmortem fetal brains will be evaluated on multiple levels, including transcriptome, proteome, and activity of noncoding regulatory elements. The objective of this aim is to define a network of genes and their regulatory elements—primarily enhancers—that are differentially active between males and females during early brain development. Aim 2 will determine whether the sex-biased regulatory network carries an increased burden of potentially pathogenic or risk associated genetic variants in developmental disorders such as SCZ and ASD. We will query available genome sequencing and GWAS datasets of probands and typical controls to determine whether the sex-biased regulatory network are enriched for rare and common inherited disease variants. Enrichment of these variants in the sex-biased regulatory network will reveal potential mechanisms by which sex-biased gene expression affects SCZ and ASD genetic risk and trajectory. The impact of this work is seeking the first detailed understanding of early sexually dimorphic brain development. The new knowledge will constitute a fundamental advance in our understanding of normal development, and will pioneer the discovery of how disease susceptibility depends on sex-biased gene regulation in the developing brain.