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(A) The structure of a guanine quartet with a stabilizing K+ in the center channel. Below, the canonical potential G4-forming sequence (PQS). G, guanine; N, any nucleotide. (B) Different G4 structural conformations: (I) intra-strand parallel; (II) intra-strand anti-parallel; (III) inter-strand anti-parallel; (IV) inter-strand parallel. (C) NMR structure of a PQS present in the human KRAS proto-oncogene promoter (RCSB PDB ID: 5IV) (176).
