The protein encoded by this gene is the human homolog of the mouse VpreB3 (8HS20) protein, and is specifically expressed in cell lines representative of all stages of B-cell differentiation. It is also related to VPREB1 and other members of the immunoglobulin supergene family. This protein associates with membrane mu heavy chains early in the course of pre-B cell receptor biosynthesis. The precise function of the protein is not known, but it may contribute to mu chain transport in pre-B cells. [provided by RefSeq, Sep 2008]