C USP45 KO U2OS cells were transiently transfected with a construct encoding FLAG‐ERCC1. Thirty‐six hours post‐transfection cells were treated with lactacystin (5 μM) for 6 h and lysed with a buffer containing 0.1 M N‐ethylmaleimide. Flag‐ERCC1 was immunoprecipitated and treated in the absence or presence of 0.1 μg of recombinant USP45 wild‐type (WT), non‐ERCC1‐binding USP45 [Asp25Ala, Glu26Ala] (AA) or catalytically inactive USP45 [Cys199Ala] (CA) for 1 h. The reactions were terminated by addition of sample buffer and analysed by immunoblot and Coomassie staining. Similar results were obtained in at least two separate experiments. 