Associates with the Ig-mu chain to form a molecular complex that is expressed on the surface of pre-B-cells. This complex presumably regulates Ig gene rearrangements in the early steps of B-cell differentiation. Interacts with IGLL1 (PubMed:17431183). Interacts with SYNV1/HRD1 (via N-terminus); this interaction leads to increased VPREB1 ubiquitination and degradation in pre-B cells, possibly through a lysosomal, not proteasomal, pathway (By similarity). Only expressed by pre-B-cells. Belongs to the immunoglobulin superfamily. VPREB1