The goal of this observational study is to attempt to find altered genetic 'switches' (mutations) in adults with advanced solid cancers who are being considered for, or are participating in, early-phase cancer clinical trials. Research has shown that cancer cells make use of key mutations to 'switch on' their growth, spread, and survival. This study will use this information to evaluate what happens to mutations over time which may help match patients to suitable clinical trials and support the development of more personalised cancer treatments. The main question it aims to answer is: Can mutations identified in various samples, including tumour samples and blood samples, help predict which patients are most likely to benefit from specific cancer treatments or clinical trials? Participants will provide consent to allow previously collected tumour samples, such as tissue from earlier biopsies or surgery, to be used for molecular analysis; allow researchers to collect and analyse clinical information about their cancer, treatments, clinical trial participation, and outcomes; be asked to provide optional research samples, which may include blood, urine, saliva, stool samples, cheek swabs, rectal swabs, tumour biopsies, and other bodily fluids when clinically appropriate; have tumour and other samples analysed to identify genetic and molecular changes that may influence cancer growth, treatment response, and treatment resistance; be informed if clinically significant findings are identified that may be relevant to their cancer care; and be followed over time through review of medical records to monitor clinical outcomes and the safety of research sample collection procedures, including biopsies. This study does not test a specific treatment. Information gained from the study may help researchers better understand cancer biology, identify new treatment targets, improve patient selection for future clinical trials, and support the development of personalised cancer therapies.