Horizontal gene transfer (HGT) is the stable transfer of genetic material from one organism to another without reproduction or human intervention.  Transfer occurs by the passage of donor genetic material across cellular boundaries, followed by heritable incorporation to the genome of the recipient organism.  In addition to conjugation, transformation and transduction, other diverse mechanisms of DNA and RNA uptake occur in nature.  The genome of almost every organism reveals the footprint of many ancient HGT events.  Most commonly, HGT involves the transmission of genes on viruses or mobile genetic elements.  HGT first became an issue of public concern in the 1970s through the natural spread of antibiotic resistance genes amongst pathogenic bacteria, and more recently with commercial production of genetically modified (GM) crops.  However, the frequency of HGT from plants to other eukaryotes or prokaryotes is extremely low.  The frequency of HGT to viruses is potentially greater, but is restricted by stringent selection pressures.  In most cases the occurrence of HGT from GM crops to other organisms is expected to be lower than background rates.  Therefore, HGT from GM plants poses negligible risks to human health or the environment.