GNEM (or HIBM), is a rare, severely debilitating disease of adult onset myopathy and progressive muscle weakness caused by a defect in the biosynthetic pathway for sialic acid (SA). Substrate replacement is a potential therapeutic strategy based on the success of replacing reduced SA and the resulting reduction of muscle disease in a relevant mouse model of the human disease. The primary objective of this Phase 2 study is to evaluate the safety of open-label 6 g/day Ace-ER in GNEM subjects with severe ambulatory impairment. The study will also assess efficacy to ensure that the full spectrum of patients with GNEM are evaluated.