Enrichment of rare codon clusters at the 5' and 3' termini of E. coli open reading frames. At non-terminal positions, 2.35% of ORFs contain a rare codon cluster. In contrast, 8.49% of ORFs contain a rare codon cluster at the 5' end, and 4.60% of ORFs contain a cluster at the 3' terminus. Only ORFs ≥ 250 windows long were evaluated. Random reverse translations (black dots) of the E. coli ORFeome show no position-specific variation in the percent of ORFs containing a rare codon cluster.