USP7 regulates the strength of interferon type I (IFN‐I) signaling. (a) The ISRE‐luciferase activity was measured in HEK293T cells transfected with control shRNA (−) or three shUSP7, together with ISRE‐luciferase and Renilla, and then treated with IFN‐α (500 IU/ml) for 18 hr. (b) The ISRE‐luciferase activity was measured in HEK293T cells transfected with Flag‐USP7, together with ISRE‐luciferase and Renilla, and then treated with IFN‐α (500 IU/ml) for 18 hr. (c) The ISRE‐luciferase activity was measured in HEK293T cells transfected with Flag‐USP7 (WT or inactive C223S mutant), together with ISRE‐luciferase and Renilla, and then treated with IFN‐α (200, 500 IU/ml) for 18 hr. (d) Quantitative‐PCR analysis of IFIT1, ISG15 and ISG54 mRNA levels in HEK293T cells transfected with shCON or shUSP7 and then treated with IFN‐α (1000 IU/ml) for 8 hr. (e) Quantitative‐PCR analysis of IFIT1, ISG15 and ISG54 mRNA levels in HEK293T cells transfected with Flag‐USP7 (WT or inactive C223S mutant) and then treated with IFN‐α (1000 IU/ml) for 8 hr. NS, not significant, *P < 0·05, ***P < 0·001 (two‐tailed unpaired Student's t‐test). All data are shown as mean ± SD of three independent replicates