Evidence for lateral gene transfer in SS9. Each P. profundum gene of ≥ 200 codons is represented in this graph by a point with co-ordinates corresponding to its codon bias relative to average gene and G+C frequency of codon site three. Genes having G+C frequency lower than 0.28 and codon bias higher than 0.35 correspond to the left horn. Genes having G+C frequency higher than 0.46 and codon bias higher than 0.35 correspond to the right horn. Genes absent in 3TCK and/or DSJ4 genomes (highlighted in black) are more frequent in horn regions (64% in left horn, 66% in right horn) respect to the middle region (10% of ORFs). The low number of strains analyzed leads to an underestimation of the laterally transferred regions in SS9 and this probably accounts for those genes localized in left and right horn which are present in all three P. profundum strains analyzed.