Myoblasts expressing low levels of SMN protein have reduced proliferation, fusion, and myotube formation, suggesting a cell autonomous defect independent of innervation.38, 39 Prenatal delay in muscle growth and maturation has been reported in severe SMA mice, as indicated by drastically smaller myotubes when the muscle was cultured in vitro. Here, SMN2 is linked to proximal spinal muscular atrophy.