In this model, control and SMA-like mice are obtained by breeding mice that are heterozygous for mouse Smn (Smn+/−) with Smn-null mice that carry two alleles with two copies of SMN2 (Smn−/−; SMN2tg/tg, total 4 copies SMN2) on a congenic FVB background (22,23). This evidence concerns the gene SMN2 and proximal spinal muscular atrophy.