The present invention provides compositions and methods for increasing proliferation, increasing cell cycle activity, and/or proliferation of postmitotic mammalian differentiated cardiomyocytes. The invention can be used to slow, reduce, or prevent the onset of cardiac damage caused by, for example, myocardial ischemia, hypoxia, stroke, or myocardial infarction in vivo. In addition, the methods and compositions of the invention can be used to enhance proliferation of differentiated cardiomyocytes in vitro and/or ex vivo, which can then be used in tissue grafting.