Autosomal-dominant familial amyotrophic lateral sclerosis (FALS) is associated with mutation in the gene that encodes Cu/Zn superoxide dismutase (SOD1). We identified a novel missense mutation of SOD-1 (Cys6Gly) in exon 1 in a Japanese woman and her family. The illness showed rapid progression similarly to the FALS with a mutation of Cys6Phe that was reported by Morita et al. (1996) (Morita, M., Aoki, M., Abe, K., Hasegawa, T., Sakuma, R., Onodera, Y., Ichikawa, N., Nishizawa, M. and Itoyama, Y., A novel two-base mutation in the Cu/Zn superoxide dismutase gene associated with familial amyotrophic lateral sclerosis in Japan. Neurosci. Lett., 205 (1996) 79-82). Mutation of the cystein at amino acid 6 might be associated with the rapid progression of ALS.