Finally, the fact that this phenomenon was not observed in the cellular assay might be caused by a spatial arrangement of the BTLA protein on the cell surface making the disulfide bridges on the BTLA surface less accessible for the tested peptides. We hypothesize that the small, compared with peptides, cysteine amino acid could reach the BTLA surface despite the spatial hindrance, hence only activity of cysteine amino acid was observed in the cellular test.