In choosing the specific sequence encoding for the FLAG marker peptide, several factors were considered. The sequence was limited to eight amino acids AspTyrLysAspAspAspAspLys . Due to this small size, the marker peptide can be encoded by a single synthetic oligonucleotide. It is known that aromatic amino acids are the major factors in antigen–antibody interactions. Therefore, Tyr was placed in position 2,flanked by charged amino acids, since the likelihood of aromatic amino acids being involved in antigenic sites is much higher than in less polar environments.