The present application relates to materials and methods for exploiting synthetic lethality and/or chemo-sensitisation in DNA damage response (DDR) pathways. In particular, the application relates to ubiquitin hydrolase protein Ubiquitin Specific Protease 4 (USP4) and its association with DDR pathways. Further, the use of USP4 inhibitors in the treatment of cancer and methods of screening is described, in particular, use of inhibitors of USP4 in the treatment of tumours defective in double-strand break repair (DSBR) and/or tumours resistant to platinum-based chemotherapy, or to sensitise, i.e. chemosensitise or radiosensitise tumours to other therapeutic agents.
