Researchers in the U.K. have overthrown the orthodox view that childhood cancers have a low mutation burden, opening up new drug targets and opportunities for repurposing existing therapies. In particular, a high mutation rate is associated with a response to cancer immunotherapy. But although PD-1 checkpoint inhibitors are approved for treating pediatric cancers with a high level of microsatellite instability mutations, in general it is thought childhood tumors are not amenable to immunotherapy.