THE candidate oncogene bcl-3 was discovered as a translocation into the immunoglobulin alphalocus in some cases of B-cell chronic lymphocytic leukaemias¹. The protein Bcl-3 contains seven so-called ankyrin repeats. Similar repeat motifs are found in a number of diverse regulatory proteins but the motifs of Bcl-3 are most closely related to those found in 1κB proteins in which the ankyrin repeat domain is thought to be directly involved in inhibition of NF-κB activity. No biological function has yet been described for Bcl-3, but it was noted recently² that Bcl-3 interferes with DNA-binding of the p50 subunit of NF-κB in vitro. Here we demonstrate that Bcl-3 can aid κB site-dependent transcription in vivo by counteracting the inhibitory effects of p50/NF-κB homodimers. Bcl-3 may therefore aid activation of select NF-κB-regulated genes, including those of the human immunodeficiency virus.