Radiotherapy can be very effective in the treatment of brain tumours, but it is associated with significant and irreversible long-term side effects, especially in children. Piao et al. transplanted oligodendrocyte progenitors (hOPCs) derived from human embryonic stem cells in the irradiated brains of young (4-week-old) rats. Injection of hOPCs in the forebrain remyelinated the brain and rescued cognitive deficits, whereas hOPC grafts in the cerebellum restored lost motor skills.