Ro5-3335 has previously been shown to improve pulmonary hypertension and retinal angiogenesis by altering vascular remodelling, endothelial to haemopoietic transition, and pulmonary macrophage activity.40,41 Given the ability of Ro5-3335 to inhibit RUNX1-dependent processes outside the heart it was important to determine that the preservation in contractile function observed in Ro5-3335 treated mice was related to a direct effect on cardiomyocytes. Here, RUNX1 is linked to pulmonary arterial hypertension.