(A) Calculated structural model of Bdecas9. The amino acid near the 5 position of the NTS is histidine. Histidine’s side chain forms a potential hydrogen bond with the 6-hydroxyl group of guanine, if the guanine is other bases, this hydrogen bond would be not formed because of the too close distance (cytosine or thymine) or the lack of a hydroxyl group (adenine). (B) Calculated structural model of Asucas9. The amino acid near the 5 position of NTS is aspartic, and it forms a potential hydrogen bond with the 4-amine group of the cytosine. if the cytosine is replaced by other bases, this hydrogen bond would be abolished because of increased distance (adenine or guanine) or the lack of an amine group (thymine). (C) Calculated structural model of Bdecas9. The amino acid near the 8 position of TS is glutamine, and it forms a potential hydrogen bond with the 6-amine group of the adenine on the TS. If the cytosine is replaced by other bases, this hydrogen bond would be not formed because of too close distance (cytosine or thymine) or the lack of an amine group (guanine). TS: target strand, NTS: non-target strand.