Radiation treatment of brain cancer can impair memory, attention and learning. Viviane Tabar at the Memorial Sloan Kettering Cancer Center in New York and her colleagues used human embryonic stem cells to make progenitor cells that form oligodendrocytes, which insulate nerve fibres, boosting the speed of electrical impulses. The team injected these cells into the brains of rats that had been exposed to radiation. The animals did better at learning and memory tasks than irradiated rats that had not received cells, and about as well as untreated rats.