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Many viruses, including retroviruses, are characterized by their specific cell tropism^1–3. Lymphadenopathy-associated virus (LAV) is a human lymphotropic retrovirus isolated from patients with acquired immune deficiency syndrome (AIDS) or related syndromes^4,5, that displays selective tropism for a subset of T lymphocytes defined by the expression of a surface glycoprotein of relative molecular mass 62,000 (62K) termed T4 (refs 6–8). This glycoprotein delineates a subset of T lymphocytes with mainly helper/inducer functions, while T lymphocytes of the reciprocal subset express a glycoprotein termed T8, have mainly cytotoxic/suppressor activities, and are unable to replicate LAV^7,9. Such a tropism may be controlled at the genomic level by regulatory sequences, as described for the human T-cell leukaemia viruses HTLV-I and -II (refs 2,3). Alternatively or concomitantly, productive cell infection may be controlled …