The T-DNA is flanked by 25 bp direct repeats. These repeats differ at only two positions between the left and right ends. Mutation or deletion studies (Hille, J., Wullems, G. and Schilperoort, R.A. (1983). Non-oncogenic T-region mutants of Agrobacterium tumefaciens do transfer T-DNA into plant cells. Plant Molecular Biology 2: 155-163) of the left and right border sequences have conclusively proved that extensive mutations of the right T-DNA border lead to an almost complete loss of T-DNA transfer capacity. However, mutations of left border only lower the transfer efficiency. These studies clearly indicate that T-strand synthesis is initiated at the right border, and proceeds in the 5′ to 3′ direction. The left border may act as initiation site of ssT-DNA , but the efficiency is much lower. It has been shown that this difference occurs because of the presence of an enhancer sequence next to the right border. This enhancer sequence is specifically recognized by VirC1 protein (Toro, N., Datta, A., Carmi, O.A., Young, C., Prusti, R.K. and Nester, E.W. (1989) The Agrobacterium tumefaciens virC1 gene products binds to overdrive, a T-DNA transfer enhancer. Journal of Bacteriology 171: 6845-6849.).