incorporation into oligonucleotides of the novel nucleoside building blocks 9, 10, and 16, carrying purine-like double H-bond-acceptor bases. These base-modified nucleosides were conceived to recognize selectively a cytosine⋅guanine (C⋅G) inversion site within a homopurine⋅homopyrimidine DNA duplex, when constituent of a DNA third strand designed to bind in the parallel binding motif. While building block