To further determine the role of cysteine residues from HVEM(23–39) peptide in the inhibition of BTLA/VEM interaction, several HVEM(23–39) analogues were designed and synthesized (Table 2). Initially, we aimed to test, if the Cys29 residue could be replaced with aromatic residue that could interact with His127 residue of BTLA to improve the interaction of the peptide. To test this, we synthesized the HVEM(23–39) C29Y peptide. To additionally probe the importance of the cysteine interactions, we replaced the Cys29 residue with the serine residue, which is of similar size and polarity. We assumed that the C29S analog could mimic the cysteine interactions but bears no free sulfhydryl group that could interact non-specifically with BTLA. We were also wondering, if even shorter HVEM-derived peptide could inhibit the interactions between BTLA and HVEM. We synthesized therefore 9 amino acid-long peptides comprising the ®-strand interface of HVEM, HVEM (31–39) fragment.