STK11 and Down syndrome: The upstream kinase that phosphorylates this regulatory phosphosite of AMPK is LKB1 (liver kinase B1), which is a tumor suppressor mutated or silenced in various sporadic cancers and in an inherited cancer susceptibility syndrome, Peutz-Jeghers syndrome (Shaw et al., 2004; Shackelford and Shaw, 2009; Hardie, 2015).