For many complex diseases, a mix of a person’s genes and their environment determines their susceptibility, with genetic influences often playing a greater role in children. Li et al. investigated how genes contribute to the development of autoimmune diseases such as type I diabetes and ulcerative colitis in children. By comparing 5000 pediatric autoimmune pediatric cases with over 36,000 healthy controls, the authors determined that genetics contributes substantially to susceptibility to autoimmune diseases. The highest heritability was observed for type 1 diabetes and juvenile idiopathic arthritis, whereas the environment played a greater role in susceptibility to lupus. Similar to autoimmunity in adults, heritability correlated highly with genetic variation in the major histocompatibility region.