Neuronal intranuclear inclusion disease (NIID) is a slowly progressive, neurodegenerative disease. NIID may affect any part of the nervous system (central, peripheral, and/or autonomic), as well as various organs. Therefore, people with NIID may have impairment of balance, movement, cognition, communication, behavior, and the ability to function independently. In general, limb weakness and ataxia are more common in children with NIID, while dementia is more common in people diagnosed in adulthood. The features of NIID result from the presence of eosinophilic intranuclear inclusions inside neurons and glial cells (abnormal masses of substances in the nuclei of cells of the nervous system). Recently, a genetic change in the NOTCH2NLC gene has been found to cause NIID.