CD79A  Cluster of differentiation CD79A also known as B-cell antigen receptor complex-associated protein alpha chain and MB-1 membrane glycoprotein, is a protein that in humans is encoded by the CD79A gene. The CD79a protein together with the related CD79b protein, forms a dimer associated with membrane-bound immunoglobulin in B-cells, thus forming the B-cell antigen receptor (BCR). This occurs in a similar manner to the association of CD3 with the T-cell receptor, and enables the cell to respond to the presence of antigens on its surface. It is associated with agammaglobulinemia-3. The mouse CD79A gene, then called mb-1, was cloned in the late 1980s, followed by the discovery of human CD79A in the early 1990s. It is a short gene, 4.3 kb in length, with 5 exons encoding for 2 splice variants resulting in 2 isoforms. CD79A is conserved and abundant among ray-finned fish (actinopterygii) but not in the evolutionarily more ancient chondrichthyes such as shark. The occurrence of CD79A thus coincides with the evolution of B cell receptors with greater diversity generated by recombination of multiple V, D, and J elements in bony fish contrasting the single V, D and J elements found in shark. CD79a is a membrane protein with an extracellular immunoglobulin domain, a single span transmembrane region and a short cytoplasmic domain. The cytoplasmic domain contains multiple phosphorylation sites including a conserved dual phosphotyrosine binding motif, termed immunotyrosine-based activation motif (ITAM). The larger CD79a isoform contains an insert in position 88-127 of human CD79a resulting in a complete immunoglobulin domain, whereas the smaller isoform has only a truncated Ig-like domain. CD79a has several cysteine residues, one of which forms covalent bonds with CD79b. CD79a plays multiple and diverse roles in B cell development and function. The CD79a/b heterodimer associates non-covalently with the immunoglobulin heavy chain through its transmembrane region, thus forming the BCR along with the immunoglobulin light chain and the pre-BCR when associated with the surrogate light chain in developing B cells.