Cu(2+)-Zn2+ superoxide dismutase (SOD-1) is a ubiquitously synthesized eukaryotic enzyme with a variety of important roles in mammalian development and physiology. With the goal of better understanding the structure and regulation of this gene, we have cloned and analyzed the genomic structure of the mouse SOD-1 gene. The murine sequence has an intron:exon organization very similar to that of the human gene, though significant diversion was observed in the promoter and 3' regions. These differences account for the observation that only a single transcript for SOD-1 is seen in mouse cells, while two transcripts are derived from the human gene. The cloning of mouse SOD-1 should allow elucidation of the regulatory characteristics of this element, as well as new and informative genetic engineering experiments.