Abstract Body: Background: Sex differences are prevalent across many human diseases, including both adult and congenital cardiovascular disease. Uncovering the mechanisms driving these sex differences is necessary to advance clinical approaches for both men and women with heart disease. KDM6A (formally UTX) is an X-chromosome-linked H3K27 demethylase that specifically removes histone H3 lysine K27 trimethylation, thereby priming transcriptional activation. Mutations in KDM6A cause Kabuki syndrome in humans, a rare congenital craniofacial disorder commonly associated with heart defects. Studies in mice have shown that Kdm6a is critical for embryonic stem cell differentiation into cardiomyocytes (CMs) and global homozygous deletion of Kdm6a leads to embryonic lethality in female, but not male, mice due to developmental heart defects. While there is substantial evidence that KDM6A plays a critical role in the heart, the sex-specific epigenetic function of KDM6A within the heart remains unknown. Methods: We have employed a CM-specific KDM6A conditional null mouse model (Kdm6a cmKO) to determine the sex-specific requirement for KDM6A in male and female hearts. Results: We find that KDM6A loss in CMs during heart development has sex-specific effects on cardiac conduction in adult mice. We also report widespread gene dysregulation in distinct pathways in male and female adult Kdm6a cmKO CMs, including opposing effects on genes involved in contractility, ion transport, and calcium ion homeostasis. Finally, we find that postnatal KDM6A loss exclusively impacts cardiac conduction in female mice and results in distinct electrocardiogram signatures compared to female Kdm6a cmKO mice. Discussion: Ongoing work aims to identify the sex-specific genomic targets of KDM6A at key developmental and postnatal time points. Overall, this research will define a sex-specific role for KDM6A in the epigenetic regulation of gene expression within the heart and will provide further insight into the biological mechanisms underlying sex differences in human heart physiology and disease.