Abstract Body: Hypertrophic cardiomyopathy (HCM) affects up to 1:500 young people worldwide. Known genetic mutations account for approximately 60% of HCM cases, with identified mutations in cardiac myosin binding protein C (MYBPC3) accounting for up to 40-50% of cases. We used CRISPR-based genome editing to create a mouse harboring a truncated MYBPC3 (MYBPC3^W1082*) based on the genetic testing of an HCM patient at UVA. We observed histologic and morphometry phenotypes consistent with human HCM. Mice carrying the homozygous mutation (MYBPC3^W1082*) display a significant increase in heart mass, CM cross-sectional area, and wall thickness at eight weeks of age in both males and females. These mice demonstrate a significant decrease in ejection fraction as early as eight weeks of age, which continues to decline until death with a median age of survival of 32 weeks. Finally, homozygous mutation results in a significant increase in fibrosis measured by Masson’s trichrome staining. Interestingly, adult murine MYCBP3^W1082* cardiomyocytes express cell cycling markers, including a substantial increase in phospho-histone 3 staining, suggesting continued cycling and endoreplication. This finding correlates with human data, which indicates a significant increase in pathways of “regulation of cell population proliferation” and “regulation of cyclin-dependent kinase activity” when comparing previously published snRNA-Seq from cardiomyocytes of HCM to non-failing controls. While the homozygous mutation results in severe disease as early as 8 weeks of age, mice carrying the heterozygous mutation display no overt cardiac or developmental defects at baseline. Unexpectedly, we determined that pregnancy as a physiological stressor elicits a significant decrease in ejection fraction and an increase in wall thickness after the birth and lactation of a single litter, which is further exacerbated with a second litter. The mice also had increased interstitial fibrosis after the second litter. These findings indicate a novel role for monoallelic MYBPC3 mutation in driving disease under stress and reveal a novel regulatory pathway in HCM, which could serve as a therapeutic target. The investigations elucidate the importance of understanding the cellular mechanisms that drive HCM in the context of a MYBPC3 mutation.