The non-Pro/Ala2 HLA-B*51:01 subpeptidome has characteristic length distribution and residue usage at P1 and PΩ. Length (A), P1 (B), and C-terminal (C) residue frequency of Pro2, Ala2 and non-Pro/Ala2 peptides are shown. One of two independent experiments is shown. Chi-squared test with Bonferroni correction used for statistics. Dashed box highlights the data with significant differences between Pro2, Ala2, and non-Pro/Ala2 peptides.