Two new members of the ABC superfamily of transporter genes have recently been identified within the Major Histocompatibility Complex in man, rat and mouse. Although the exact function of these genes is not known, they have been shown to be necessary for the presentation of peptides derived from the degradation of cytoplasmic protein antigens to the cellular immune system. For this reason they have been named TAP1 and TAP2 (for Transporter associated with Antigen Presentation). Each gene encodes one membrane spanning domain and one region homologous to the ATP binding domains that characterise the superfamily. The two proteins encoded by the TAP genes form a complex that is localised to the membranes of the endoplasmic reticulum and cis-golgi. Their most likely function is to transport short peptides, that lack signal sequences, from the cytoplasm to the endoplasmic reticulum, although the evidence for this is still indirect.