This is an open-label multi center study to evaluate the safety of Aceneuramic Acid Extended Release (Ace-ER) Tablets for patients with GNE Myopathy.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.