A subset of pedigrees with dominant inheritance of familial amyotrophic lateral sclerosis have mutations in superoxide dismutase 1. Initial studies suggested that disease-linked mutations impaired superoxide dismutase 1 activity, which is consistent with the notion that disease results from increased oxidative injury. However, results of recent cell culture and transgenic studies demonstrate that mutant proteins retaining high levels of superoxide dismutase 1 activity cause motor neuron degeneration; elevating the level of wild-type superoxide dismutase 1 does not cause disease. These findings suggest that the familial amyotrophic lateral sclerosis phenotype may occur through other mechanisms that can now be explored in model systems.