Illustration of chimeric antigen receptor (CAR) T cells (blue) attacking cancerous white blood cell precursors (dark pink) in a case of chronic lymphoblastic leukaemia (CLL). CLL is a cancer in which excess white blood cell precursors called lymphoblasts are produced in the bone marrow. The chronic form develops very slowly, and symptoms may not appear for years. Treatment can involve immunotherapy, chemotherapy and radiotherapy. CAR-T therapy is a type of immunotherapy that genetically modifies a patients own T cells, a form of white blood cell, to recognise and destroy cancer cells.