SMN1 and proximal spinal muscular atrophy: At present, the most commonly used SMA models are termed Smn−/−; SMN2 (Hsieh-Li et al., 2000; Monani et al., 2000), Smn−/−; SMN2; SMNΔ7/Δ7 (Le et al., 2005) and Smn2B/− mice (Hammond et al., 2010; Bowerman et al., 2012a) and range from severe (death within the first post-natal week) to intermediate phenotypes (average lifespan of 30 days).