GSK-3β is phosphorylated and inhibited by a number of kinases. In the absence of upstream kinases, unphosphorylated GSK-3β is active, and it phosphorylates and inactivates downstream targets such as glycogen synthase to modulate metabolism or phosphorylates transcription factors and targets them for degradation. Juhaszova et al. (9) suggest that phosphorylation and inactivation of GSK-3β also reduce activity of the MPT. Most GSK-3β substrates are phosphorylated by another kinase prior to phosphorylation by GSK-3β (12). GS, glycogen synthase; p70^s6k, p70 ribosomal S6 kinase; RSK, p90 ribosomal S6 kinase; ILK, integrin-linked kinase; WNT, homologues of Wingless; P, phosphorylation.