A 9-year-old female patient experienced progressive weakness and myalgias of shoulders and back of several months duration. Her medical history was notable for spina bifida in association with a Chiari type II malformation and hydrocephalus. Developmental motor milestones were delayed whereby walking began at age 2. She had mild bowel and bladder dysfunction. At presentation, her neurological exam was notable for weak shoulder adduction, hip and knee flexion and she demonstrated a partial Gower's maneuver. A muscle biopsy showed dystrophic changes and immunohistochemical findings of a Duchenne's mosaic which was confirmed by DNA analysis. The proposed pathogenesis in this case is unfavourable lyonization, which was corroborated by X-inactivation studies.