STO-609 is a selective and cell-permeable inhibitor of the Ca 2+/calmodulin-dependent protein kinase kinase (CaM-KK), with Ki values of 80 and 15 ng/mL for recombinant CaM-KKα and CaM-KKβ, respectively. STO-609 inhibits AMP-activated protein kinase kinase (AMPKK) activity in HeLa cell lysates with an IC50 ~0.02 g/ml.