GSK-3 (Glycogen Synthase Kinase-3) is a serine/threonine kinase, consisting of two isoforms, GSK-3α and GSK-3β. They localize in both the cytoplasm and nucleus and are widely expressed in various cells and tissues. GSK-3 is involved in regulating glycogen synthesis, cell cycle, cell differentiation, and signaling pathways such as Wnt/β-catenin and PI3K/Akt. By phosphorylating multiple substrates, GSK-3 plays a role in cell growth, death, and neuronal function. Its activity is negatively regulated by upstream signals such as insulin and growth factors. Aberrant activation of GSK-3 is closely associated with the development of Alzheimer's disease, bipolar disorder, and cancer, making it a potential therapeutic target.