The protein encoded by this gene belongs to the immunoglobulin superfamily and is expressed selectively at the early stages of B cell development, namely, in proB and early preB cells. This gene encodes the iota polypeptide chain that is associated with the Ig-mu chain to form a molecular complex which is expressed on the surface of pre-B cells. The complex is thought to regulate Ig gene rearrangements in the early steps of B-cell differentiation.