TAPBP and neoplasm: In addition to changes in the tapasin expression level, other parameters determining the level of peptide presentation can also be subject to change during the course of an immune response to a tumor or virus: (a) source protein levels (e.g., an oncoprotein) may vary over orders of magnitude; (b) levels of expression of antigen processing enzymes and TAP may vary and impact peptide supply; (c) individual anchor amino acids that comprise the agretope within a peptide may mutate to enhance or diminish binding to MHC (peptide off-rate), without necessarily affecting the epitope.