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(A) Chemical structure of tetrahydrobiopterin; (B) complex of a RNA duplex with tetrahydrobiopterin (PDB code: 4LVX); (C) chemical structure of pemetrexe; and (D) complex of a RNA duplex with pemetrexed (PDB code: 4LVY). The orange structure represents the small molecule; the purple structure represents the backbone of nucleic acid; the blue and red atoms in the molecule represent nitrogen and oxygen atoms, respectively.
