This study is researching an experimental drug called ALN-CIDEB, also referred to as "study drug". The study is focused on participants with metabolic dysfunction-associated steatotic liver disease (MASLD) (Part A) and metabolic dysfunction-associated steatohepatitis (MASH) (Part B). MASLD and MASH are long-lasting liver conditions caused by having too much fat in the liver. The aim of the study is to see how safe and tolerable the study drug is. The study is looking at several other research questions, including: * What side effects may happen from taking the study drug * How the study drug works to change liver fat content * How much study drug and study drug metabolites (byproducts of the body breaking down the study drug) are in the blood at different times