As we have previously observed with other MHC/Cyscontaining peptide complexes [19, 23] , complexes did not readily form unless the cysteine of the S598 peptide (RCQIFANI) was modified with L-a-aminobutyric acid (Aba, an isostereomer of cysteine). The Aba-modified peptides maintained immunogenicity since a higher frequency of splenic CD8+ T cells from JHMVimmune mice reacted to Aba-modified S598 peptide, relative to unmodified S598 peptide ( Figure S1A ).