MHC class I intron 2 sequences in humans ( HLA ) and chimpanzees ( Patr ). Only the polymorphic nucleotide positions are indicated. Identity to the consensus sequence (depicted at the top) is indicated by dashes. Substitutions and inserts are depicted by the conventional one-letter code; deletions are marked “x”. For instance, “-/A” indicates that differences in a particular sequence have been reported in the literature. The brackets indicate the division of the intron 2 alleles into lineages, and this is based on phylogenetic analysis. (The figure is adapted from De Groot N.G. et al., PNAS 99, 11748-11753, 2002).