Abstract: Splice variants of known TAP1 and TAP2 proteins, which are involved in translocation of antigen peptides into the endoplasmic reticulum for complexing with MHC class I molecules and eventual display on the cell surface, are disclosed. A fully sequenced and characterized splice variant of TAP2, designated TAP2iso, is shown to form functional heterodimers with TAP1 and to exhibit a peptide specificity that differs from previously studied TAP1/TAP2 transporter proteins. The discovery of splice variant TAP subunits alters the prior theory of immune response and introduces a mechanism for diversification of antigen display to the CD8-positive T cells of the immune system. Methods for diagnosis and treatment of diseases or conditions associated with abnormal TAP splice variant expression, or of expanding the reptertoire of antigen peptides to which an individual's immune system is capable of responding, are also disclosed.