Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Glycogen synthase kinase 3 (GSK-3) is a serine/threonine protein kinase that mediates the addition of phosphate molecules on certain serine and threonine amino acids in particular cellular substrates. The phosphorylation of these other proteins by GSK-3 usually inhibits the target protein (as in the case of glycogen synthase and NFAT; the target protein is also called the "substrate"). In mammals GSK-3 is encoded by two known genes GSK-3 alpha and beta.