Measurement of released inorganic triphosphate (PPPi) from hydrolysis of ara-CTP (200 µM) by recombinant SAMHD1 (0.35 µM) in the presence of GTP (200 µM) and a titration of different non-hydrolysable dNTP analogues (dNTPαS) in the enzyme-coupled malachite green assay. Error bars indicate s.e.m. of two independent experiments performed in triplicate and quadruplet. 