Intracellular dNTP measurements using a primer-extension assay in SAMHD1+/+ THP-1 cells treated for 4 or 24 h with either 50 µM HU (middle panel) or 2.5 nM 3-AP (right panel), ratios of dCTP-to-dATP were calculated. Bars indicate mean values of three independent experiments, error bars indicate s.e.m. Statistical analyses were done using unpaired two-tailed t-tests: **, P < 0.01. 