Mice do not harbor the SMN2 gene as it is unique to humans, but by introducing low copies of the human SMN2 gene into the Smn−/−background, researchers were able generate a mouse model (Smn−/−;SMN2) mimicking the severe SMA phenotype (Monani et al., 2000). This evidence concerns the gene SMN2 and proximal spinal muscular atrophy.