Protein disulfide isomerase or PDI is an enzyme in the endoplasmic reticulum in eukaryotes that catalyzes the formation and breakage of disulfide bonds between cysteine residues within proteins as they fold. PDI contains four thioredoxin-like domains, two of which retain the canonical CXXC motif. The reduced (that is, dithiol) form of PDI is able to catalyse a reduction of mispaired thiol residues of a particular substrate, acting as an isomerase. Another major function of PDI relates to its activity as a chaperone; i.e., it aids wrongly folded proteins to reach a correctly folded state without the aid of enzymatic disulfide shuffling. PDI helps load antigenic peptides into MHC class I molecules. These molecules (MHC I) are related to the peptide presentation by antigen-presenting cells in the immune response.