This level of Smn protein is well above that of Smn+/− mice that have a normal lifespan [1] or heterozygous SMA model mice [(SMN2)Ahmb89+/+;Smn+/−; Jax strain 5024], which have no motor neuron loss and a normal lifespan [54]. This evidence concerns the gene SMN2 and proximal spinal muscular atrophy.