AMPK (AMP-activated protein kinase) is a key cellular component that regulates energy homeostasis. When the AMP/ATP ratio is increased due to energy depletion, AMPK is phosphorylated at its ?1 and ?2 catalytic subunits and activated. Studies showed that the tumor suppressor LKB1 phosphorylates and activates AMPK in mammals. MO25? (mouse protein 25?), also called CAB39 (calcium binding protein 39), is a component of the LKB1 complex in vivo. MO25?/CAB39 does not directly interact with LKB1; instead, it is physically associated with STRAD?. Together MO25?/CAB39 and STRAD? help anchor LKB1 in the cytoplasm. MO25?/CAB39 stablizes the LKB1-STRAD? complex and further increases the catalytic activity of LKB1.