The right ventricle (RV) is involved in systemic circulation in the fetal mammalian heart but quickly transitions to being solely responsible for pulmonary circulation after birth when the left ventricle (LV) becomes the systemic ventricle. To handle the increased workload, LV growth greatly outpaces that of the RV during postnatal stages. However, the molecular basis for this differential growth pattern between the 2 chambers is largely unknown. In this issue of the JCI, Yokota et al. reveal that the p38 mitogen-activated protein kinase (MAPK)/IRE1α/XBP1 axis specifically controls postnatal RV growth by suppressing cell cycle regulatory genes.