SMN2 and proximal spinal muscular atrophy: In order to determine the extent of heterogeneity in motor neuron pathology occurring between distinct pools of motor neurons in a severe mouse model of SMA (Smn−/−;SMN2; [22]), we initially quantified NMJ pathology in ten anatomically distinct skeletal muscles from various body regions at a late-symptomatic time-point (postnatal day 5; P5).