A T-DNA binary system is a pair of plasmids consisting of a binary plasmid and a helper plasmid. The two plasmids are used together to produce genetically modified plants. They are artificial vectors that have both been created from the naturally occurring Ti plasmid found in Agrobacterium tumefaciens. The binary vector is able to replicate in both E. coli and Agrobacterium tumefaciens.
Systems in which T-DNA and vir genes are located on separate replicons are called T-DNA binary systems. T-DNA is located on the binary vector (the non-T-DNA region of this vector containing origin(s) of replication that could function both in E. coli and in Agrobacterium tumefaciens, and antibiotic-resistance genes used to select for the presence of the binary vector in bacteria, became known as vector backbone sequences). The replicon containing the vir genes became known as the vir helper. Strains harboring this replicon and a T-DNA are considered disarmed if they do not contain oncogenes that could be transferred to a plant.
There are several binary vector systems that differ mainly in the plasmid region that facilitates replication in Agrobacterium. Commonly used binary vectors include: