Genetic testing revealed an MEN1 variant of uncertain significance (rs2959656).<h4>Diagnoses</h4>As the clinical findings fulfilled the diagnostic criteria, MEN1 was confirmed.<h4>Interventions and outcomes</h4>His cognitive decline and behavioral symptoms improved after treatment for syndrome of inappropriate secretion of antidiuretic hormone without dementia-specific medications. The gene discussed is MEN1; the disease is Down syndrome.