A. tumefaciens causes crown gall disease by transferring T-DNA from its Ti (tumour-inducing) plasmid into the plant cell nucleus, where it integrates stably. For biotechnology, tumour-causing genes (oncogenes) on T-DNA are replaced with the gene of interest, while vir genes on the Ti plasmid (needed for transfer) are retained. This disarmed system provides a natural mechanism for stable nuclear transformation of dicotyledonous plants.