Mutations in superoxide dismutase cause a subset of familial amyotrophic lateral sclerosis and provoke progressive paralysis when expressed in mice. After retrograde transport to the spinal cord following injection into muscles, an adeno‐associated virus carrying a gene that encodes a small interfering RNA was shown to target superoxide dismutase messenger RNA for degradation. The corresponding decrease in mutant superoxide dismutase in spinal motor neurons preserved grip strength. This finding provides proof of principle for the selective reduction of any neuronal protein and supports intramuscular injections of a small interfering RNA–encoding virus as a viable therapy for this type of familial amyotrophic lateral sclerosis. Ann Neurol 2005;57:773–776