The RP105 is a novel, murine B cell surface molecule that is expressed on mature B cells but not on pre-B or T cells. The RP105 is a 150 kDa type I transmembrane protein, and a member of the leucine-rich repeat protein family. Tandem repeats of a leucine-rich motif in the extracellular domain are thought to be involved in processes such as cell adhesion or receptor-ligand interactions. An antibody against RP105 protects B cells from irradiation- or dexamethasone-induced apoptosis, and drives them to proliferate. B cells activated by RP105-ligation arrest their growth and undergo apoptosis when the antigen receptor is engaged. Thus, RP105 is a signal transduction molecule and seems to have a role in regulation of B cell growth and death.