Transgenic mice over-expressing mutant human SOD1 have increased SOD1 activity and a loss of motor neurons that models human ALS, and mice carrying a mutant SOD1 transgene (tgSOD1G85R) on a normal mouse background compared with the same transgene in a SOD1 null background showed no change in survival. The gene discussed is SOD1; the disease is amyotrophic lateral sclerosis.