Immunoglobulin-binding protein 1 (IGBP1),also known as ALPHA-4,was originally identified as a signal transduction molecule coprecipitating with MB1 (CD79a) protein of the B-cell antigen receptor (BCR) complex. IGBP1 has been characterized as an essential regulator of the catalytic activity of protein phosphatase 2A (PP2A),which is a ubiquitous phosphatase with broad substrate specificity and diverse cellular functions.