Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a condition that affects brain development and function. Individuals with this condition have an enlarged brain (megalencephaly) and an abnormality of the white matter in the brain (leukoencephalopathy). White matter consists of nerve fibers covered by a fatty substance called myelin that promotes the rapid transmission of nerve impulses. In MLC, the myelin is swollen and contains numerous fluid-filled pockets (vacuoles). Over time, the swelling decreases and the myelin begins to waste away (atrophy). Leukoencephalopathy can lead to abnormal muscle tensing (spasticity), difficulty coordinating movements (ataxia), cysts in the brain (subcortical cysts), abnormal muscle tone (dystonia), swallowing difficulties, mild to moderate intellectual disabilities, speech difficulties, seizures, and difficulties walking. There are three types of MLC, which are distinguished by their signs and symptoms and genetic cause. Type 1 is caused by genetic changes in the MLC1 gene. Types 2A and 2B are caused by genetic changes in the HEPACAM gene. MLC types 1 and 2A are inherited in an autosomal recessive manner, while type 2B is inherited in an autosomal dominant manner. In some cases of MLC, the cause is unknown.