Transgenic introduction of SMA intragenic missense mutations—specifically SMN1(A2G) [199], SMN1(A111G) [200], SMN1(D44V) [201], SMN1(T74I) [201] and SMN1(Q282A) [201]—into severe SMA mice (two copies of SMN2 on an mSmn zulligygous background) improves the motor phenotype of severe SMA mice but does not completely ameliorate the SMA phenotype in these mice. The gene discussed is SMN2; the disease is proximal spinal muscular atrophy.