MHC class I complexes are present at the surface of almost all cell types in jawed vertebrates, and display antigenic peptide fragments derived from endogenously synthesised proteins. This process allows screening of the cells protein repertoire, and facilitates detection and destruction of virally infected or cancerous cells by CD8 + cytotoxic T-cells. MHC class I complex assembly occurs in the endoplasmic reticulum. Here the Class I molecule Is loaded with an antigenic peptide, facilitated by a group of ER resident proteins which constitute the peptide loading complex. The formation of interchain disulphides between constituents of the peptide loading complex (heavy chain, ERp57, PDI, tapasin) suggests a role for redox reactions during peptide loading, a role further inferred by the presence of an intrachain disulphide bond within the peptide binding groove in the α2 domain of heavy chain.