Most strains produce one type of neurotoxin, but strains producing multiple toxins have been described. C. botulinum producing B and F toxin types have been isolated from human botulism cases in New Mexico and California.The toxin type has been designated Bf as the type B toxin was found in excess to the type F. Similarly, strains producing Ab and Af toxins have been reported. Evidence indicates the neurotoxin genes have been the subject of horizontal gene transfer, possibly from a viral 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, indicating that the C. botulinum acquired the toxin genes quite far in the evolutionary past.