The low SOD1 mutant protein level (∼10% of WT in homozygotes) may be sufficient to elicit a degree of motor neuron degeneration but not the levels of loss seen in ALS, and may at least partially explain why loss of motor neurons within the sciatic motor pool appears not to progress between 15 and 52 weeks of age. The gene discussed is SOD1; the disease is amyotrophic lateral sclerosis.