A Study to Evaluate ALN-CIDEB in Adult Participants With Metabolic Dysfunction-Associated Steatotic Liver Disease or With Metabolic Dysfunction-Associated Steatohepatitis (MASLD/MASH)

Key Inclusion Criteria:

• Part A: 18 to 55 years at Screening Visit 1 with MASLD, at Screening Visit 1 Part B: 18 to 65 years at Screening Visit 1 with a diagnosis of MASH, at Screening Visit 1
• Body Mass Index (BMI) ≥30 kg/m2 and ≤40 kg/m2 at Screening Visit 1
• Controlled-Attenuation Parameter (CAP) ≥285 dB/m by FibroScan during screening as described in the protocol
• Liver fat content ≥8.5% by MRI-PDFF during screening
• If on anti-hypertensive and/or lipid lowering medications and/or glucose lowering medications, must be on generally stable dose(s) for at least 12 weeks prior to screening and no changes to the dose(s) are anticipated during the study
• Part B: A diagnosis of MASH documented in the participant's medical history, or a clinical suspicion of MASH based on non-invasive biomarkers (eg, evidence of fatty liver on imaging and elevated liver enzymes) and clinical risk factors, including having a history of 2 or more elements of metabolic syndrome, as defined in the protocol
• Part B: Screening percutaneous liver biopsy NAFLD Activity Score (NAS) ≥3 and fibrosis stage, as defined in the protocol

Key Exclusion Criteria:

• Known historical or current diagnosis of portal hypertension or cirrhosis based on clinical assessment, imaging, and/or liver biopsy
• Known historical or current diagnosis of other forms of chronic liver disease, as defined in the protocol
• Prior or current suspected or known drug-induced liver injury within 1 year prior to screening
• History of liver transplant, current placement on a liver transplant list, or Model for End-stage Liver Disease (MELD) score >12
• Contraindication to MRI examinations, such as persons with cardiac pacemaker and implants made of metal, severe claustrophobia, size restrictions, or other contraindications for MRI
• Liver stiffness measurement, laboratory parameter assessment, estimated Glomerular Filtration Rate (GFR), and evidence of uncontrolled hypertension, as defined in the protocol
• Evidence of Human Immunodeficiency Virus (HIV) infection, Hepatitis B Virus (HBV) infection, or Hepatitis C Virus (HCV) infection during screening, as described in the protocol
• History of Type 1 Diabetes
• Bariatric surgery, including any procedures to revise, reverse, or remove any previous bariatric surgery interventions, within approximately 5 years prior to randomization or planned during the study period

NOTE: Other protocol-defined inclusion/exclusion criteria apply.