In mammals, the Rel/NF-κB family of transcription factors consists of five members RelA (p65), RelB, c-Rel, NF-κB1 (p105/p50), and NF-κB2 (p100/p52). The major goal of first project is to understand whether NF-κB2/p100 and RelB function in B-cell development by using p100-/- mice (still producing p52 but not p100) as a model system. Not only an arrested mainstream B-cell development at the onset of B-lineage commitment, but also an impaired marginal zone B-cell generation, was found in p100-/- mice in a cell-intrinsic manner. The elevated RelB DNA-binding activity was restored to wild-type levels when one allele of the relB gene was deleted in p100-/- mice, correlated with the rescued B-cell development. The impaired B-cell development in p100-/- mice was likely due to the reduced expression of B-lineage transcription factors EBF and Pax5. Moreover, the increased RelB function determined the fate of B lymphocytes versus myeloid cells, likely due to the contribution of enhanced C/EBPα expression in p100-/- B-cell precursors. Thus, the findings unravel a specific role of alternative NF-κB activation pathway (NF-κB2/p100 and RelB) with respect to B-lymphocyte development. The activation of NF-κB through lymphotoxin β receptor (LTβR) is critical for secondary lymphoid organogenesis. LTβR signaling induces both classical (p50/RelA) and alternative (p52/RelB) complexes. To in vivo dissect the individual contribution of each NF-κB activation pathway downstream of LTβR signaling with a specific focus on the secondary lymphoid organogenesis, one approach is to generate mouse models, in which relB or relA can be selectively inactivated in LTβR-expressing cells. Towards this goal, three mouse strains need to be established, including mice harboring floxed relB (relBflox/flox) or relA alleles (relAflox/flox, generated) and mouse strain with Cre recombinase activity in LTβR-expressing cells. In this study, the generation of relBflox/flox mice is in the stage of germline transmission. Four ltbr-cretg founder lines were generated by pronucleus injection of cre cDNA under the control of the ltbr gene promoter. In three founder lines, the tissue expression pattern of cre mRNA was consistent with that of endogenous ltbr mRNA.