A Study to Evaluate ALN-6400 in Healthy Volunteers and Patients With Hereditary Hemorrhagic Telangiectasia (HHT)

Inclusion Criteria - Part A: * Is a healthy adult volunteer Part B: * Is an adult patient with a clinical diagnosis of HHT Exclusion Criteria - Part A: * Has alanine aminotransferase (ALT) or aspartate aminotransferase (AST) \> upper limit of normal (ULN) * Has known human immunodeficiency virus (HIV) infection; or known current or chronic hepatitis C virus or hepatitis B virus infection * Has an estimated glomerular filtration (eGFR) of \<90 mL/min/1.73m\^2 at screening Part B: * Has ALT or AST \>2×ULN * Has total bilirubin \>1.5×ULN * Has eGFR of \<30 mL/min/1.73m\^2 at screening Parts A and B: * Is not willing to comply with the contraceptive requirements during the study period Note: other protocol defined inclusion / exclusion criteria apply