1. AIDS Virus Research Unit, National Institute for Communicable Diseases, Johannesburg, South Africa.
2. Department of Zoology, University of Oxford, Oxford, UK.
3. Chris Hani Baragwanath Hospital, Johannesburg, South Africa.
4. Department of Virology, University of the Free State, Bloemfontein, South Africa.
5. Present address: Department of Virology, Inkosi Albert Luthuli Central Hospital, Durban, South Africa.
6. Present address: Department of Molecular Medicine and Haematology, University of the Witwatersrand Medical School, Johannesburg, South Africa.