We have characterized a genomic clone of the mouse equivalent of alpha1-antitrypsin (alpha1-antiprotease) a gene that is expressed in liver. There are at least three other genomic sequences similar to the gene expressed in mouse liver and all appear to be located on chromosome 12. The duplicated regions might include genes that are also expressed in the liver and/or in other tissues. Comparison of the sequences of the mouse alpha1-antitrypsin sequence expressed in liver with sequences of alpha1-antitrypsins from other mammals reveals great homology in the coding regions, particularly in the amino acids at the active site, including the neighboring methionine and serine thought to play a key role in the protease inhibition.