text: Nuclear levels of NF-kappaB correlate with syncytium-forming capacity of 8e51 cells, expressing a defective HIV virus. 
  The double NF-kappaB site identified in the LTR of the human immunodeficiency virus-1 (HIV-1) has been demonstrated to be necessary for efficient viral transcription. In this report we present the characterisation of NF-kappaB subunits engaged in complexes binding to the HIV-1 NF-kappaB site in human 8e51 T-cells, that harbour a defective HIV-1. At least four different specific NF-kappaB complexes are present in the nucleus of these cells. With the use of specific antibodies we have determined the composition of each complex using electrophoretic mobility shift assays. The results show the presence of several NF-kappaB family members, with the transactivating RelA being engaged in multiple complexes. The importance of NF-kappaB complexes in viral functions has been established comparing the level of NF-kappaB DNA-binding complexes with syncytia-forming activity of 8e51 cells. In fact, 8e51 cells that had almost lost their syncytia-forming capacity were found to contain at least 10 times less active NF-kappaB DNA-binding complex than the actively fusing cells. The correlation is specific as the level of at least three other transcription factors did not change. 