CD79 consists of two proteins, CD79a (mb-1) and CD79b (B29). CD79a recognizes the Ig-alpha protein, and CD79b recognizes the Ig-beta protein of the B-cell antigen component of the B lymphocyte antigen receptor. CD79b is a multimeric complex that includes the antigen-specific component, surface immunoglobulin (Ig). Surface Ig non-covalently associates with two other proteins, Ig-alpha and Ig-beta, which are necessary for the expression and function of the B-cell antigen receptor. In normal B-cell differentiation, CD79b (B29) is first expressed in cells that have Ig μ chains and remains expressed throughout B-cell differentiation up to the plasma cell stage. Cells from most chronic B-cell disorders, for example, most B-cell lymphomas and B-cell prolymphocytic leukaemias, are CD79b positive. However, CD79b is either absent or weakly expressed in neoplastic B cells from chronic lymphocytic leukaemia (CLL) and hairy cell leukaemia.