Distribution of purifying selection sites and sites of intracodon recombination. Enrichment of sites under purifying selection among codon sites showing evidence of intracodon recombination in dengue virus serotypes. The total number of sites that show evidence of recombination is shown within the box but outside the two circles. Of these, the number of sites showing intracodon recombination and sites under purifying selection is shown in solid and dashed line circles. The number of sites under both selection and intracodon recombination is shown in the overlapping region of the two circles. The p-value above each serotype (shown as DENV-1, DENV-2, DENV-3 and DENV-4) represents the statistical significance of a hypergeometric test to reject the null assumption that the observed enrichment pattern is a random event.