Cover. Non-Watson-Crick base pairs can be classified geometrically by their interacting edges (Watson-Crick, Hoogsteen or sugar) and the relative orientations of the glycosidic bonds (cis or trans) into 12 families, six of which are shown-those involving at least one sugar edge. Upper left panel, cis Watson-Crick/sugar edge; lower left, trans Watson-Crick/sugar edge; upper center, cis Hoogsteen/sugar edge; lower center, trans Hoogsteen/sugar edge; upper right, cis sugar edge/sugar edge; lower right, trans sugar edge/sugar edge. For the Watson-Crick/sugar edge and sugar edge/sugar edge pairs, a third base forms a canonical Watson-Crick base pair with one of the bases implicated in the non-Watson-Crick interaction. See the paper by Leontis et al. in this issue [Nucleic Acids Res. (2002) 30, 3497-3531].
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