GSK-3 is a serine/threonine protein kinase that phosphorylates and inactivates glycogen synthase. GSK-3 has been implicated in the regulation of cell fate and in the Wnt signaling pathway. GSK-3 plays an important role in the PI3 kinase and Akt mediated cell survival pathways, and its activity can be inhibited by Akt-mediated phosphorylation at Ser21 of GSK-3α and Ser9 of GSK-3β.