Selectivity of four methods for the identification of compositional atypical genes. Two sets were analysed consisting of genes VCA0010 to VCA0230 (control group) and genes VCA0271 to VCA0491 (belonging to the integron island) from chromosome two of V. cholerae. For each gene, the indicators codon usage contrast (CU), δ* difference, dicodon usage (DC) and h
(gene) (as introduced here) were determined as described, the values were accumulated set-wise in histograms. Any position on a curve gives on the two axes the fraction of genes below the corresponding cut-off value.