CpG-dinucleotide content in HLA-DRB1 exons 1–6, based on the dataset including the entire DRB1 coding region (49 sequences). A, the observed CpG-dinucleotide content. B, the observed CpG level (as in A) divided by the mathematically estimated CpG content (based on the total G+C level). Mean ± sem is shown. The ratios were separately calculated for each allele and then averaged.