B-lymphocyte-specific membrane protein that plays a role in the regulation of cellular calcium influx necessary for the development, differentiation, and activation of B-lymphocytes. Functions as a store-operated calcium (SOC) channel component promoting calcium influx after activation by the B-cell receptor/BCR. Forms homotetramers. Interacts with the heavy and light chains of cell surface IgM, the antigen-binding components of the BCR. Constitutively associated with membrane rafts. Phosphorylated. Mice have severely impaired T-cell independent antipolysaccharide antibody responses (PubMed:20038800). B-cell development is predominantly normal in CD20-deficient mice but calcium influx following CD19 or IgM ligation is reduced (PubMed:8450218). Belongs to the MS4A family.