SMN1 and muscular atrophy: However Spinal muscular atrophy mutant mice that die soon after birth (low copy SMN2+/+;Smn -/-) preclude detailed analysis of pathogenic mechanisms and preclinical drug testing [28] However, this disease severity can be tempered to intermediate and mild phenotypes by adding additional transgenes that express various wild-type isoforms or weak mutant forms of SMN [29].