Alignment of the amino acid sequence of the murine Cyp21 with the human CYP21 protein adapted from Ref. 45 . Amino acid sequences are designated by the universal single-letter code and residues are numbered relative to the first N-terminal methionine. Hyphens indicate sites where comparable amino acids are missing. The shaded areas indicate single amino acid exchanges in the H-2^aw18 Cyp21 protein.