Human farnesyl pyrophosphate synthase (hFPPS) plays a key role in the prenylation of small GTPases, such as RAS and RAP 1A, which are intimately involved in oncogenesis. An allosteric pocket of the enzyme has been of particular interest as a therapeutic target, however, its natural biological function has been (until now) unknown. The teams of Berghuis (Biochemistry) and Tsantrizos (Chemistry) have just reported that the catalytic product of hFPPS, farnesyl pyrophosphate (FPP), bind to this pocket and locks the enzyme in a conformationally inactive state. Therefore, this allosteric binding site offers an exquisite mechanism for controlling the intracellular levels of isoprenoid biosynthesis in vivo. This research recently appeared in Nature Communications.