Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a very rare disorder of the brain. Individuals who have MLC are typically either born with or develop a large head early in the first year of life. Early development, such as walking, is typically normal or mildly delayed; however, a slow deterioration of motor skills starts in early childhood, and most children affected with this disorder will require a wheelchair sometime during their teenage years. Intellectual disability is mild and speaking and swallowing difficulties can also develop. Most of these individuals will have epileptic seizures that can be controlled with medicine. Some individuals have died in adolescence, and some have survived into their 40s. There is no cure for MLC; however medical surveillance and care may help to improve some symptoms and overall condition of life. MLC is caused by pathogenic variants in the MLC1 gene.