Typing Alleles of TAP1 and TAP2

TAP1 and TAP2 (for transporter associated with antigen processing) are two ATP-binding bassette (ABC) transporter genes located within the class II region of the human major histocompatibility complex (MHC) (1 ). Their protein products form a heterodimer within the membrane of the endoplasmic reticulum (ER), which transports oligopeptides from the cytoplasm into the lumen of the ER. Once within the ER, peptides are bound by MHC class I molecules and then transported to the cell surface where they can be recognized by cytotoxic T cells (2 ). Because of their role in antigen processing and their location within the class II region of the MHC, these genes have attracted attention as possible disease susceptibility loci (3 –8 ). In the rat, alleles of TAP2 transport different subsets of peptides (9 ), although this does not appear to be the case in humans.