The paper cited in References by Franzolin et al. (2013) presents evidence that dNTP pool sizes are controlled not only at the level of dNTP synthesis (RNR), but also at the level of dNTP breakdown, the latter catalyzed by an enzyme called SAMHD1. Speculate on the reaction catalyzed by SAMHD1. Speculate on allosteric control mechanisms that might regulate SAMHD1 activity.