 Starting in the 1990s, it became clear that GC-biased gene conversion was a major factor—previously unanticipated—in affecting GC content in diploid organisms such as mammals.Similarly, although it may be the case that bacterial genome composition strongly reflects GC and AT biases, the proposed mutational biases have not been demonstrated to exist.Indeed, Hershberg and Petrov suggest that mutation in most bacterial genomes is biased toward AT, even when the genome is not AT-rich.