Erratum
In the editing process, an error was introduced in the first paragraph of a letter to Science from Mikulas Popovic which appeared in the 16 August News & Comment section (p. 730). In that paragraph, the human immunodeficiency virus (HIV-l)IIIB should have been identified as the human T cell lymphotropic virus (HTLV)-IIIB/83. In the description of productive infection of neoplastic CD4+ T cell lines with HIV-1 isolates at the end of paragraph 8, reference 7 should have included a paper by E.-M. Fenyö et al. [J. Virol. 62, 4414 (1988)].