As observed in chicken cell lines, Cbl-b was required for ligand-induced clustering on the cell surface. In these cell line studies, Cbl played a role in coupling the BCR to the motor protein dynein. Dynein has also been implicated in endocytic trafficking indicating that the observed defects in clustering and endocytic trafficking could reflect a common underlying mechanism. The unique ability of Cbl-b to facilitate receptor endocytic trafficking mapped to the Cbl-b carboxyl-terminal tail that contains the UBA domain. One well-described difference between Cbl-b and c-Cbl is that the Cbl-b UBA domain can bind Ub while the c-Cbl UBA cannot. Ubiquitin binding mediates Cbl-b dimerization and increases E3 ligase activity. However, the ability of Cbl-b to enable BCR endocytic trafficking to late endosomes was not dependent on the Cbl-b ubiquitin-binding motif. In chicken B cells, c-Cbl is recruited to the BCR and is required for BCR surface clustering. Chicken c-Cbl is also predicted to not bind Ub, and therefore, there is likely another unidentified functional domain, conserved between chicken c-Cbl and human and murine Cbl-b, that enables BCR endocytic trafficking. Previous publications have focused on Cbl-b as a negative regulator of lymphocytes. Mice deficient in Cbl-b develop an autoimmune syndrome associated with lymphocytic infiltrates in multiple organs, and they are highly susceptible to experimental autoimmune encephalomyelitis and collagen induced arthritis. Cblb2/2 T cells do not need CD28 costimulation for activation and cannot be tolerized. These phenotypic changes have been related to Cbl-b-mediated regulation of PI3K and PLCc1, respectively. In B cells, Cbl-b negatively regulates EX 527 signaling through CD40 and has been demonstrated to bind and ubiquitinylate multiple proximal BCR signaling components. Our observation that Cbl-b is recruited to the aggregated BCR complex provides a mechanism by which Cbl-b could gain access to many of these signaling substrates. However, given the multiple negative signaling functions ascribed to Cbl-b, the phenotype of Cblb2/2 mice is relatively mild. Autoimmunity does not develop until after six months of age, and potentially pathogenic immune complex deposits in glomeruli are only observed in some aged mice in which both Cbl and Cblb had been targeted in B cells. By ELISA, anti-dsDNA antibodies are detected in Cblb2/2 mice. However, such ELISAs can be falsely positive and are not as relevant as Crithidia luciliae immunofluorescence assays. In fact, in MRL/ Mplpr/lpr mice, TLR9 deficiency greatly diminishes anti-dsDNA responses as measured by IF but not as measured by ELISA. Thus, the available evidence indicates that the intrinsic B cell defect in Cblb2/2 mice is not severe. We propose that this is because Cbl-b is a complex molecule with both negative and positive effects on peripheral B cell activation.