The specificity of TAP proteins was first investigated by trapping peptides in the ER using glycosylation. TAP binds to 8- to 16-residue peptides with equal affinity, while translocation is most efficient for peptides that are 8 to 12 residues long.
Initial research on TAP protein specificity focused on capturing peptides within the ER using glycosylation.  TAP proteins bind to peptides of 8 to 16 amino acids with equal strength, and translocation is most effective when peptides are 8 to 12 residues in length.