No changes in left ventricular (LV) wall thickness were observed in Jak1cKO mice compared to control genotypes (Fig. 4A, E) but LV dilation occurred by 6 months of age (Fig. 4F) and was accompanied by significant impairment of systolic function (Fig. 4G–I, Supplemental Table S2), revealing dilated cardiomyopathy in aged mice with cardiomyocyte-specific loss of JAK1. The gene discussed is JAK1; the disease is dilated cardiomyopathy.