B-Cell Antigen Receptor Complex-Associated Protein Alpha Chain(CD79a), also known as CD79α, IgM-α, Ig-α and mb-1, which is a signaling component of the preB cell receptor (preBCR). Covalently linked CD79a and CD79b heterodimer constitutes the B cell antigen receptor (BCR) with membrane immunoglobulin (mIg) in a non-covalent association. CD79a is required in cooperation with CD79b for initiation of the signal transduction cascade activated by binding of antigen to the B-cell antigen receptor complex (BCR), leading to internalization of the complex, trafficking to late endosomes and antigen presentation. CD79a is also required for BCR surface expression and for efficient differentiation of pro- and pre-B-cells.