Pathways for oxidative folding in the ER of Mus musculus. Mice contain two ERO1 isoforms (α and β) that use PDI and/or PDI-like proteins (PDI(s)) to oxidize substrates. ERO1-β has a specific impact on the oxidative folding of insulin in the mouse pancreas (Zito et al., 2010). VKOR and QSOX proteins may also facilitate oxidative folding in the ER, although their connections to folding in the ER lumen are less well established. VKOR, QSOX, or alternative unidentified pathways in the ER, including small molecules, may allow for the oxidative folding of IgM in the absence of ERO1 activity.