A study in Nature Genetics has published genetic variants associated with rare seizures that occur in some children who have been vaccinated against measles, mumps and rubella (MMR). The results of this study represent an important first step toward understanding the etiology of seizures in children of European ancestry who have been vaccinated with the MMR vaccine. Fever is a common reaction to injections of vaccines such as the MMR vaccine that are derived from live viruses whose virulence has been reduced. In some rare cases, such vaccines can lead to the development of fever-related seizures, which are called febrile convulsions and typically last 1 to 2 minutes, with no long-term risk of neurological effects at this time. In a small percentage of cases of MMR vaccine, young children develop seizures in the second week after vaccination. The cause of how fever caused by vaccination leads to seizures has been unknown. Bjarke Feenstra et al. compared genetic variants in 1,300 children who had received the MMR vaccine and triggered a febrile convulsive response, 2,000 children who had not received the MMR vaccine but had febrile convulsions, and 5,800 children who had no history of epilepsy. They identified two genetic variants associated with MMR vaccine-induced febrile convulsions: IFI44L and CD46, which play an important role in the innate immune response – the innate immune response responsible for processing the initial immune response triggered by infection. expression is enhanced following measles infection, and CD46 is thought to be associated with the immune response triggered by the MMR vaccine.