The antigen receptor on B cells (B cell receptor [BCR]) consists of two noncovalently associated modules. Immunoglobulin genes created somatically during B cell development encode the antigen-specific component of the receptor. The Igα/β heterodimer, encoded by the mb-1 and B29 genes, is necessary to escort the receptor complex to the plasma membrane. Following antigen engagement of the BCR, Igα/β nucleates signal transduction and promotes endocytosis of bound antigen for intracellular degradation and presentation to helper T-cells. In this review, we outline the discovery of the mb-1 gene; summarize results from other laboratories on the function of Igα/β in B cells; and conclude with our recent studies, which indicate that mb-1 is not a B-lineage-restricted gene as originally proposed.