The NF?B transcription factor was originally identified as a protein complex consisting of a DNA binding subunit and an associated protein. The subunit is functionally related to c-Rel p75 and Rel B p68. The p50 subunit was initially believed to be a functionally unique protein derived from the amino-terminus of a precursor designated p105. A cDNA has been isolated that encodes an alternative DNA binding subunit of NF?B. It is synthesized as a protein that is expressed in a variety of cell types and, like p105, undergoes cleavage to generate its NF?B subunit, in this case a protein designated p52 (previously referred to as p49). In contrast to p50 derived from p105, p52 acts in synergy with p65 to stimulate the HIV enhancer in transiently transfected Jurkat cells.