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Fig. 3. Distribution of Shiga toxin (Stx) genes in E. coli O157 clades. (A) The frequency of 528 O157 strains that were classified into one of nine clades based on SNP genotyping, ranked from left to right in the histogram by decreasing frequency. The four most common clades were clades 2 (47.6%), 8 (25.4%), 3 (10.6%), and 7 (7.3%). (B) Distribution of Shiga toxin variants (stx1, stx2, and stx2c) among 519 of the 528 O157 strains organized into nine clades. The percentage of PCR assay positive strains overall is given in parentheses.