RUNX1 and myocardial infarction: These data demonstrate that marked differences in SR-mediated calcium release/uptake between BZ and RZ cardiomyocytes that are known to contribute to whole heart contractile dysfunction in the first week post-MI, are evident as early as 1-day post-MI in both C57BL/6J and cre-negative littermate controls and are Runx1-dependent.