Human T-lymphotropic virus type I and II (HTLV-I and -II) are retroviruses associated with neurologic degeneration and leukemia in a small part of infected individuals. HTLV-II is now spreading among intravenous drug users in Southern Europe, threatening the blood banks. The origin of these human retroviruses and the reasons for their observed genomic stability are not known. We have access to a unique African reservoir of endemic HTLV-II among Bambuti pygmies from Zaire. Sequencing this viral genome will allow us to investigate the origin of HTLV-II. The reverse transcriptase of this virus will be cloned and expressed to be able to investigate its fidelity as a possible reason for the extreme genetic stability of this retrovirus. Possibly, a reverse transcriptase with a high fidelity could result from this research, which could become a useful tool in molecular biology research.