Triplex formation has been observed between an oligodeoxyribonucleotide duplex and an oligodeoxyribopyrimidine which contains IV4-(3-acetamidopropyl) cy-tosine, a novel base analog which selectively interacts with a CG base pair of the target duplex.

Oligodeoxyribopyrimidines can interact with the ho-mopurine strand of an oligodeoxyribopurine-oligo-deoxyribopyrimidine duplex through the formation of C+< GC and·· base triads. 1-4 The presence of a single CG or· base pair in theotherwise purine-pyrimidine sequence motif of the target duplex can be accommodated by naturally occurring bases in the third strand, although such “mismatches” generally lead to overall reduced stability of the triplex. 2-6 An exception to this is the observation that G can recognize a single· base