Gene transfer is the transfer of one chromosome or genome segment to another chromosome or genome. Gene transfer occurs in nature in both prokaryotes and eukaryotes. There are three routes for the entry of DNA: transformation, transduction, and conjugation. Gene transfer can also be used in medicine, for example, to treat disease by supplying patients with therapeutic genes. There are also many possible ways to transfer genes. Most involve the use of a vector, such as a specially modified virus that can take the gene along when it enters the cell.