A disulphide-linked heterodimer, consisting of mb-1 (or CD79a) and B29 (or CD79b) polypeptides, is noncovalently associated with membrane-bound immunoglobulins on B cells to constitute the B cell Ag receptor. CD79a first appears at pre B cell stage and persists until the plasma cell stage where it is found as an intracellular component. CD79a is found in the majority of acute leukemias of precursor B cell type, in B cell lines, B cell lymphomas, and in some myelomas. It is not present in myeloid or T cell lines.