Researchers have uncovered how embryonic livers accumulate an important energy molecule even though they lacks the key enzyme responsible. In adults, the liver stores glycogen, a sugar polymer that provides a steady supply of blood glucose when needed (e.g. during fasting). Glycogen production is controlled by an enzyme called glucokinase (GK), and mutations resulting in too much or too little GK will lead to hypo- and hyper-glycemia, respectively.