Miller-Dieker syndrome is a condition characterized by a pattern of abnormal brain development called lissencephaly. Normally, the surface of the brain (cerebral cortex) has folds and grooves. Lissencephaly causes the surface of the brain to be abnormally smooth, with fewer folds and grooves. In people with Miller-Dieker syndrome, lissencephaly is typically associated with severe intellectual disabilities, developmental delays, weak muscle tone (hypotonia), and seizures. Seizures usually begin in the first few months of life.~People with Miller-Dieker syndrome often have distinctive facial features that include a prominent forehead; a sunken appearance in the middle of the face (midface hypoplasia); a small, upturned nose; low-set and abnormally shaped ears; a small jaw; and a thick upper lip. Some individuals with this condition also grow more slowly than their peers. Less often, affected individuals have heart problems, kidney abnormalities, or an opening in the wall of the abdomen (an omphalocele) that allows the abdominal organs to protrude through the navel. Because of these severe health issues, most individuals with Miller-Dieker syndrome do not survive beyond early childhood.