AbstractWhen expressed in the absence of light chains, secretory Ig‐µ chains (µs) undergo endoplasmic reticulum associated degradation (ERAD). This process involves the recognition of terminally misfolded or unassembled molecules, their retro‐translocation across the ER membrane and ubiquitination for degradation by cytosolic proteasomes. The molecular components of the ERAD pathway and their coordination remain largely unknown. Here we employed co‐immunoprecipitation, silencing or over‐expression assays to show that SEL1L and HRD1 are involved in the degradation of unassembled Ig‐µs, but have minor effects on another substrate, TCR‐α. SEL1L and HRD1 localize in the early secretory apparatus and are induced by ER stress and during B cell differentiation, concomitantly with the onset of massive IgM secretion. These findings reveal a role for SEL1L and HRD1 in IgM quality control. J. Cell. Physiol. 215: 794–802, 2008. © 2008 Wiley‐Liss, Inc.