TAPBP and neoplasm: We therefore tested this prediction experimentally and varied peptide supply for a number of peptides of known affinity over a range that might be encountered in vivo for viral or tumor antigens (we previously estimated the maximum copy number of the fluorescent-Ub-peptide fusion protein, the source for the peptide supply, to reach a maximum of 108 molecules per cell [(34) Supplementary Figure S1B, E, F], which is similar to the maximum copy number per cell of some proteins (41, 42)), then measured the tapasin bonus for individual peptides.