A tumour inducing (Ti) plasmid is a plasmid found in pathogenic species of Agrobacterium, including [[Agrobacterium tumefaciens|A. tumefaciens]], A. rhizogenes, A. rubi and A. vitis. Evolutionarily, the Ti plasmid is part of a family of plasmids carried by many species of Alphaproteobacteria. Members of this plasmid family are defined by the presence of a conserved DNA region known as the repABC gene cassette, which mediates the replication of the plasmid, the partitioning of the plasmid into daughter cells during cell division as well as the maintenance of the plasmid at low copy numbers in a cell. The Ti plasmids themselves are sorted into different categories based on the type of molecule, or opine, they allow the bacteria to break down as an energy source. The presence of this Ti plasmid is essential for the bacteria to cause crown gall disease in plants. This is facilitated via certain crucial regions in the Ti plasmid, including the vir region, which encodes for virulence genes, and the transfer DNA (T-DNA) region, which is a section of the Ti plasmid that is transferred via conjugation into host plant cells after an injury site is sensed by the bacteria. These regions have features that allow the delivery of T-DNA into host plant cells, and can modify the host plant cell to cause the synthesis of molecules like plant hormones (e.g. auxins, cytokinins) and opines and the formation of crown gall tumours. Because the T-DNA region of the Ti plasmid can be transferred from bacteria to plant cells, it represented an exciting avenue for the transfer of DNA between kingdoms and spurred large amounts of research on the Ti plasmid and its possible uses in bioengineering. Nomenclature and classification The Ti plasmid is a member of the RepABC plasmid family found in Alphaproteobacteria. These plasmids are often relatively large in size, ranging from 100kbp to 2Mbp. They are also often termed replicons, as their replication begins at a single site. Members of this family have a characteristic repABC gene cassette. Another notable member of this family is the root inducing (Ri) plasmid carried by A. rhizogenes, which causes another plant disease known as hairy root disease. A key feature of Ti plasmids is their ability to drive the production of opines, which are derivatives of various amino acids or sugar phosphates, in host plant cells. These opines can then be used as a nutrient for the