Intracellular ara-CTP (upper panel) and dCTP:dATP ratio (lower panel) in the indicated cell lines following the indicated treatment determined using HPLC-MS/MS. SAMHD1-/- THP-1 cells were treated with 500 nM ara-C and SAMHD1+/+ THP-1 cells were treated with either solvent, 500 nM ara-C or a combination of 500 nM ara-C and an RNRi (HU, 50 µM; dF-dC, 10 nM; 3-AP, 150 nM) for 24 h prior. Values relative to mean ara-CTP amounts in ara-C treated SAMHD1+/+ THP-1 cells shown (indicated by dashed line). Circles, columns and error bars correspond to individual values, means and s.e.m. of at least three experiments performed independently. 