The article by Jeffrey G. Lawrence and Howard Ochman explores the process of amelioration in bacterial genomes, focusing on the rates of change and exchange of horizontally transferred genes. Despite wide variation in overall GC content among bacterial species, the genes within a species' genome are relatively similar in base composition. Horizontal gene transfer introduces novel sequences that initially reflect the donor genome's base composition but over time, these sequences adapt to match the recipient genome's composition due to mutational processes. This process is evident in genes involved in host-cell invasion by enteric bacteria and can be modeled to predict the time required for foreign DNA to resemble native DNA. Using a 1.43-megabase continuous sequence of the *Escherichia coli* chromosome, the authors estimate that over 600 kb of horizontally transferred protein-coding DNA exists, accumulating at a rate of 31 kb per million years. This rate suggests that *E. coli* and *Salmonella enterica* lineages have gained and lost more than 3 megabases of novel DNA since their divergence. The study also highlights the importance of directional mutation pressure in shaping base composition and the use of nucleotide composition and codon usage patterns to infer the ancestry of chromosomal genes.The article by Jeffrey G. Lawrence and Howard Ochman explores the process of amelioration in bacterial genomes, focusing on the rates of change and exchange of horizontally transferred genes. Despite wide variation in overall GC content among bacterial species, the genes within a species' genome are relatively similar in base composition. Horizontal gene transfer introduces novel sequences that initially reflect the donor genome's base composition but over time, these sequences adapt to match the recipient genome's composition due to mutational processes. This process is evident in genes involved in host-cell invasion by enteric bacteria and can be modeled to predict the time required for foreign DNA to resemble native DNA. Using a 1.43-megabase continuous sequence of the *Escherichia coli* chromosome, the authors estimate that over 600 kb of horizontally transferred protein-coding DNA exists, accumulating at a rate of 31 kb per million years. This rate suggests that *E. coli* and *Salmonella enterica* lineages have gained and lost more than 3 megabases of novel DNA since their divergence. The study also highlights the importance of directional mutation pressure in shaping base composition and the use of nucleotide composition and codon usage patterns to infer the ancestry of chromosomal genes.