The lack of motor neuron disease in SOD1 knock-out mice [1] and the normal enzymatic activity conferred by some of the SOD1 mutants [2,3] suggest a cytotoxic gain of function similar to the disease causing proteins in other neurodegenerative disorders like Alzheimer’s disease and Parkinson’s disease. The gene discussed is SOD1; the disease is early-onset autosomal dominant Alzheimer disease.