This gene encodes a chaperone for assembly of lysosomal vacuolar ATPase. Required for the assembly of the V0 complex of the vacuolar ATPase (V-ATPase) in the endoplasmic reticulum. Associates with the V0 complex of the vacuolar ATPase (V-ATPase). MEAX is a childhood-onset disease characterized by progressive vacuolation and atrophy of skeletal muscle. It is inherited in recessive fashion, affecting boys and sparing carrier females. Onset is in childhood, and patients exhibit weakness of the proximal muscles of the lower extremities, progressing slowly to involve other skeletal muscle groups over time.