The rel family of genes encodes transcription factors, such as the v-rel oncoprotein and cellular transcription complexes (eg NF-kappa B), consisting of c-rel and related polypeptides. The expression of these genes is correlated most closely with lymphoid cell differentiation and growth stimulation in a variety of cell types. Similarly, rel family proteins control a number of genes involved in lymphoid cell growth and differentiation. Thus, it is not surprising that mutations in rel genes have been implicated directly and circumstantially in a number of lymphoid malignancies, as have a number of other cellular and viral transcription factors (see also Cleary M, Hayman M and Beug H, this issue and reviewed in Rabbitts, 1991). In addition, rel proteins are likely to be involved in T cell diseases caused by the human retroviruses HTLV-I and HIV-1. Therefore, rel proteins could serve as targets for anti-viral or anti-cancer therapies.