SMN2 and proximal spinal muscular atrophy: The C+/+ mouse model (Jackson Lab; FVB.129(B6)-Smn1tm5(Smn1/SMN2)Mrph/J) is the mildest genetic model of SMA and exhibits low body weight with very mild impairment of a subset of motor functions, but has a normal life span (Osborne et al, 2012).