Gabathuler, R et al. report that introduction of the TAP1 gene into TAP-negative tumor cells changes the mode of antigen presentation to resemble that of TAP1-positive RMA-S cells, as indicated by the fact that a virally-derived antigen can be efficiently presented on the surface of TAP1-possitive but not TAP-negative tumor cells [14]. The gene discussed is TAP1; the disease is neoplasm.