Average protein bit score for strong and weak operon gene clusters. A box plot showing the different gene clusters ranked according to average pair-wise bit score of the protein sequences (BitScore) normalised against alignment length (AliLen). The legend text shows the median score of each group (weak operon 0.79 bits, strong operon 0.65 bits). Ribosomal genes are not included. When they are included the numbers are 0.81 and 0.75, respectively.