GSK3 (Glycogen Synthase Kinase-3) is a ubiquitously expressed, highly conserved serine/threonine protein kinase found in all eukaryotes. Originally identified as a regulator of glycogen metabolism, GSK3 acts as a downstream regulatory switch for numerous signaling pathways, including cellular responses to WNT, growth factors, insulin, receptor tyrosine kinases (RTK), Hedgehog pathways, and G-protein-coupled receptors (GPCR). It is involved in a wide range of signal transduction cascades involving cellular processes, ranging from glycogen metabolism, cell development, gene transcription, and protein translation to cytoskeletal organization, cell cycle regulation, proliferation, and apoptosis. GSK3 is localized predominantly in the cytoplasm but is also found in the nucleus. Its subcellular localization is changed in response to stimuli.