Western blot analysis

Cells were lysed with RIPA buffer containing protease inhibitor (Roche). Protein concentration was measured using the bicinchoninic acid method (Thermo Scientific). Total protein was separated by 8–10% SDS–PAGE and transferred using the iBlot western blotting system (Life Technologies). Primary antibodies against human and mouse PARP14 (1:250, HPA01206 Sigma-Aldrich), human and mouse PARP9 (1:250, ab53796, Abcam), human and mouse STAT1 (1:1,000, #9172, Cell Signaling), phosphorylated STAT1 (1:1,000, #9167, Cell Signaling), human and mouse STAT6 (1:2,000, #9362, Cell Signaling), mouse (1:1,000, ab54461, Abcam) and human (1:2,000, #9361, Cell Signaling) phosphorylated STAT6 and human and mouse β-actin (1:5,000; Novus) were used. For secondary antibodies, we used anti-mouse (1:1,000–5,000, A4416, Sigma) and rabbit (1:1,000–5,000, NA934-1ML, GE Healthcare Life Sciences) IgG antibodies. Protein expression was detected using Pierce ECL Western Blotting substrate reagent (Thermo Scientific) and ImageQuant LAS 4000 (GE Healthcare). Uncropped images of western blots are demonstrated in Supplementary Figs 18–20.