Figure 1.

Alignment of 443 residues of the amino acid sequence of xanthine dehydrogenase in seven species. Previously known intron positions are numbered (as in humans) above the sequences and denoted by shading: over two residues when the site falls between two codons, or over one residue when the codon is split, in which case the number in parentheses refers to the base in the triplet just before the intron. Black shading denotes novel inserted introns, labeled A, B, and C. Sources for published sequences are as follows: Drosophila melanogaster (14, 19), Calliphora vicina (20), Homo sapiens (21), and Aspergillus nidulans (22). Human intron 24 is at the same position as Drosophila intron 2.