FAD-dependent sulfhydryl oxidase that catalyzes disulfide bond formation in the endoplasmic reticulum lumen in parallel to ERO1. O2 + 2 R'C(R)SH = H2O2 + R'C(R)S-S(R)CR' Homodimer. Interacts with the substrate protein PDI1, forming transient intermolecular disulfide bridges. Present with 259 molecules/cell in log phase SD medium.