P100, which is encoded by NF-kappaB2, inhibits Rel dimers. It can also be processed into p52, one of the DNA binding sub-units of NF-kappaB/Rel factors. Several p100 C-terminal truncations that result from gene rearrangements are associated with lymphomagenesis. Here, we characterized a new p100 mutant that we termed p100HB. It originates from a point-mutation that generates a premature stop-codon, and thus the protein lacks the last 125 amino acids. We have detected p100HB in several human tumor cell lines. The truncated protein is mainly unprocessed, and although it still binds Rel dimers, it has reduced inhibitory potency compared to p100 and translocates into the nucleus. Thus, p100HB may be associated with deregulated NF-kappaB/Rel functions. (C) 2003 Elsevier Inc. All rights reserved.