Cancers are life threatening because they migrate within the body, spreading far from their point of origin. This process – metastasis – hijacks tissues and compromises their critical functions. When they reach this stage, most cancer clones will be robust and resistant to treatment, whether that be radiotherapy, chemotherapy or immunotherapy. So, in a sense, it is resistance that is the major stumbling block to successful treatment. Those exceptional cancers that are curable, even when disseminated (childhood acute lymphoblastic leukaemia, testicular cancer and choriocarcinoma) retain sensitivity.