Tumours with DNA repair defects, such as those from BRCA1/BRCA2-mutation carriers, respond very well to certain chemotherapies and to new targeted drugs named PARP inhibitors (PARPi). We have developed a new test based on the visualisation of a DNA repair protein named RAD51 that enables to establish if tumour cells have DNA repair defects, beyond mutations in BRCA1/BRCA2. This project aims to establish the predictive value of the RAD51 test in four major cancers types (breast, ovarian, prostate and endometrial) and if the test provides functional evidence for the interpretation of genetic variants of unknown significance. In addition, we will improve the test to enable its implementation in the clinic and perform an economic evaluation to compare our test versus current selection criteria for PARPi therapeutic indications.