cDNA for human mitochondrial superoxide dismutase (MnSOD or SOD2) was placed downstream of a human beta actin promoter and a 5' flanking sequence. The construct contained intron 1 sequence of the human beta actin gene to allow RNA splicing to occur. Line 66 was established. RNA blot analysis showed a 1.4 kb human MnSOD mRNA expressed in the mouse heart. Enzymatic activity in the heart is increased 325% relative to wild-type.