An Open Label Phase 2 Extension Study of Higher Dose Sialic Acid-Extended Release (SA-ER) Tablets and Sialic Acid-Immediate Release (SA-IR) Capsules in Patients With Glucosamine (UDP-N-acetyl)-2-Epimerase (GNE) Myopathy

Inclusion Criteria: * Enrollment in, and successful completion of the UX001-CL201 (NCT01517880) protocol OR (for 10 treatment naïve subjects): * Have a confirmed diagnosis of GNE Myopathy * Aged 18 -65 years of age, inclusive * Able to walk ≥ 200 meters and \< 80% of predicted normal during the 6-Minute Walk Test (6MWT; orthotics and assistive devices allowed) * Must be willing and able to provide written, signed informed consent after the nature of the study has been explained, and prior to any research-related procedures * Must be willing and able to comply with all study procedures * Sexually active subjects must be willing to use an acceptable method of contraception while participating in the study * Females of childbearing potential must have a negative pregnancy test at Baseline and be willing to have additional pregnancy tests during the study. Females considered not of childbearing potential include those who have been in menopause for at least two years, or have had tubal ligation at least one year prior to Baseline, or who have had total hysterectomy Exclusion Criteria: * Use of any investigational product (other than SA-ER tablets) to treat GNE myopathy * Ingestion of N-acetyl-D-mannosamine (ManNAc) or similar SA-producing compounds * Pregnant or breastfeeding at Baseline or planning to become pregnant (self or partner) at any time during the study * Has had any hypersensitivity to SA or its excipients that, in the judgment of the investigator, places th…