The N-(2-aminoethyl)glycine backbone unit of PNA has been derivatized with phenylacetic acid and acetic acid moieties to produce monomers for incorporation into PNAs. As expected, oligomers containing these residues form PNA-DNA double helices that are destabilized compared to the unsubstituted duplexes. However, unlike the analogous substitutions in DNA, these residues show very little sequence specificity. (C) 1999 Elsevier Science Ltd. All rights reserved.