ERp27 (Endoplasmic Reticulum protein 27.7 kDa) is a homologue of PDI (protein disulfide-isomerase), localised to the Endoplasmic Reticulum. The structure of ERp27 has been solved by both X-ray crystallography and NMR spectroscopy, showing it to be composed of two thioredoxin-like domains with homology to the non-catalytic b and b' domains of PDI. The function of ERp27 is unknown, but on the basis of its homology with PDI it is thought to possess chaperone activity.