In genetics, complementary DNA (cDNA) is DNA synthesized['sinθisaiz] from a mature mRNA template in a reaction catalyzed['kætəlaiz] by the enzyme reverse transcriptase and the enzyme DNA polymerase. cDNA is often used to clone eukaryotic[ju:,kæri'ɔtik] genes in prokaryotes[prəu'kæri:əut]. When scientists want to express a specific protein in a cell that does not normally express that protein (i.e., heterologous[,hetə'rɔləɡəs] expression), they will transfer the cDNA that codes for the protein to the recipient cell. cDNA is also produced by retroviruses 逆转录酶病毒 (such as HIV-1, HIV-2, Simian Immunodeficiency Virus, etc.) which is integrated into its host to create a provirus.