A 5-year-old boy with Down's syndrome of mild phenotype is described. Chromosome studies revealed that the karyotype of the proband was 46,XY,rec(21),dup q,inv(21) (p11.2q22.1)mat, and the segment 21q22.1----21qter was trisomic. The erythrocyte superoxide dismutase-1 (SOD-1) was found to be normal, and so we conclude that SOD-1 excess is not necessarily observed in patients with Down's syndrome caused by partial 21 trisomy. It is suggested that the gene for SOD-1 is located on the more proximal segment of the sub-band 21q22.1.