One particular class of DNA regulatory elements, the enhancers or activators, can, relatively independently of distance and orientation, dramatically increase the transcriptional activity of homologous and heterologous promoters located in cis (see refs 1-3 for reviews, also refs 4-6). Sequence differences between various heterologous enhancers may explain their apparent host- and/or tissue-specific action. Furthermore, differences in the transcriptional control elements may contribute to viral tropism. At least for murine leukaemia virus isolates, thymotropism and leukaemogenicity have been attributed to alterations within the viral long terminal repeat, which harbours their enhancers and other transcriptional control elements. We report here the identification of a viral enhancer element possessing a very restricted tissue range. The enhancer is active in all human cells of the haematopoetic system tested, but not in cells of fibroblast or epithelial origin.