Using a positional scanning combinatorial peptide library approach to perform an unbiased analysis of the specificity of SLA-1*0401 and human-pig chimeric MHC class I molecules Using human β 2 m to support folding, the recombinant SLA-1*0401 and human-pig chimeric MHC-I molecule were tested for peptide binding. We have previously described how PSCPL can be used to perform an unbiased analysis of MHC-I molecules (Stryhn et al. 1996) . A PSCPL consists of 20 sublibraries for each position where one of each of the 20 natural amino acids have been locked and all other positions contain random amino acids. Analyzing how much of each PSCPL sublibrary is needed to support MHC-I folding (see examples in Fig. 6 ) and comparing each sublibrary with a completely random library, the effect of any amino acid in any position can be examined and expressed as a RB value. Further, an AP value calculated as the sum of squared deviations of RB values for each position can be used to identify the most prominent anchor position (see "Materials and methods" for calculations). Thus, the specificity of a nonamer binding MHC-I molecule can be analyzed comprehensively with 9×20+1 completely random library=181 sublibraries (Stryhn et al. 1996) .