Prostate cancer (PrCa) is one of the commonest cancer in men in the Western world, with over 40,000 new cases diagnosed each year in the United Kingdom (UK). Research studies have identified several genetic changes that are thought to increase the risk of developing prostate cancer. Some of these genetic changes occur in deoxyribonucleic acid (DNA) repair genes. The BARCODE 2 study is formed of two parts that aim to investigate how having genetic changes in DNA repair genes can affect response to treatment. In part 1 of the study, the investigators will invite men with metastatic castration resistant prostate cancer (mCRPC) who have not had genetic testing before to join the study by initially undergoing genetic screening within the study. If a pathogenic mutation is confirmed in one of these genes, patients will be given the option to proceed to part 2 of the study. In part 2 of the study, men with mCRPC who are known to be carriers of a mutation in DNA repair gene(s) will be assessed for eligibility for treatment on the study. The aim of the study will be to determine how patients with mCRPC and a germline mutation in a DNA repair gene(s) respond to platinum chemotherapy. This study will help researchers to investigate the platinum sensitivity of prostate tumours that have developed due to a germline mutation in a DNA repair gene. This study will provide data to use in a larger clinical trial of platinum chemotherapy based on patients' germline genetic signature and/or tumour genetic profile.