MN1 C-Terminal Truncation (MCTT) syndrome is a genetic disorder caused by a change in the MN1 gene. A genetic change in the MN1 gene (also called a mutation or a pathogenic variant) has been linked to intellectual disability (ID), language delay and facial similarities among the affected individuals. More specifically, a brain malformation called rhombencephalosynapsis (RES) may be seen on the brain scan. This is a rare disorder and the exact number of cases in the population (prevalence) is unknown.