A Study to Evaluate Mivelsiran in Adult Participants With Early-Stage Down Syndrome-Associated Alzheimer's Disease (DS-AD)

Inclusion Criteria: * Clinical diagnosis of Down-Syndrome (DS) associated with trisomy 21 * Positive amyloid PET scan * Cognitively stable in the opinion of the investigator * Seizures must be well controlled with no occurrence of seizures in the 6 months prior to screening Exclusion Criteria: * Has severe intellectual disability (ID) * Has a history of DS regression disorder * Has alanine aminotransferase (ALT) or aspartate aminotransferase (AST) \>2×upper limit of normal (ULN) at Screening * Has estimated glomerular filtration rate (eGFR) \<45 mL/min/1.73m\^2 at Screening * Has recently received an investigational agent * Has had treatment with amyloid-targeting antibody * Comorbidities such as obstructive sleep apnea and hypothyroidism that are not well controlled Note: other protocol defined inclusion / exclusion criteria apply