Evidence indicates the neurotoxin genes have been the subject of horizontal gene transfer, possibly from a viral (bacteriophage) source. This theory is supported by the presence of integration sites flanking the toxin in some strains of "C. botulinum". However, these integrations sites are degraded (except for the C and D types), indicating that the "C. botulinum" acquired the toxin genes quite far in the evolutionary past. Nevertheless, further transfers still happen via the plasmids and other mobile elements the genes are located on.