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Summary Horizontal gene transfer (HGT) is characterized by a stable transfer of genetic information from one organism to another. This kind of genetic exchange occurs mainly between prokaryotic organisms (bacterial conjugation), whereas cases of HGT between prokaryotic and eukaryotic organisms are extremely rare. Some reports describe gene transfer between Escherichia coli and Saccharomyces cerevisiae cells that resembles bacterial conjugation. However, the only example of gene transfer between prokaryotic cells and the cells of higher eukaryotes is the case of T-DNA transfer from Agrobacterium to plant cells. The ability of Agrobacterium to transfer a fragment of its DNA to plant cell provides a powerful tool for plant biotechnology, and therefore the Agrobacterium-mediated DNA transfer is one of the most commonly used techniques of plant transformation. In early days this method was restricted only to the dicotyledonous plants, since it was believed that Agrobacterium could infect dicots solely. This was due to the use of tumor formation tests as indication of bacterial infection. Later, it was found out that although Agrobacterium indeed induced tumors only on dicotyledonous plants, it was able to infect monocotyledonous plants as well, but without tumor formation. The host range of Agrobacterium is not restricted to plants only. Agrobacterium is able to transfer DNA also to other bacterial species from the same family, Rhizobiaceae, as well as to other microorganisms, such as yeasts S. cerevisiae, filamentous fungi or cultivated mushrooms. Most recently, transfer of DNA from Agrobacterium to human cells has been documented.