The dNTP supply system. Thick lines are fluxes, thin solid lines are activations, thin dashed lines are inhibitions. Key enzymes are described in the text. An RNR s-site mediated large positive feedback loop ATP → dCTP → dUMP → dTTP → dGTP → dATP terminates when dATP binds the R1 a-site to inhibit all 4 RNR reductions. Models of enzymes of this system will eventually be useful in anticancer drug dose time course optimizations .