A cell-permeable bis-indolo (indirubin) compound that acts as a highly potent, selective, reversible, and ATP-competitive inhibitor of GSK3α/β (IC50 = 5 nM); inhibition of GSK activates the Wnt-signaling pathway and sustains pluripotency in human and mouse ESCs; maintains self-renewal in human and mouse embryonic stem cells as well as induces the differentiation of neonatal cardiomyocytes