This multicenter, randomized, placebo-controlled study will evaluate the efficacy and safety of zodasiran subcutaneous (SC) injection in subjects 12 years of age and older with genetically or clinically diagnosed Homozygous familial hypercholesterolemia (HoFH). After completion of the double blind (DB) treatment period subjects will be eligible to continue in the optional open-label extension (OLE) period of the study. All placebo subjects who opt to continue will transition to active drug during the OLE Period.