The crystal structure of the PNA (peptide nucleic acid) oligomer H-Lys-HalG-AlaG-HalC-AlaG-HalC-AlaC-Lys-NH(2) (PNA1, amino acids with D-configuration are underlined, Ala = alanyl, Hal = homoalanyl) has been determined by ab initio direct methods and refined against 1.0 Å data. The asymmetric unit consists of a tetrameric cage with almost ideal Watson-Crick C-G base pairing of all the guanine and cytosine side-chain substituents. Each PNA strand has a 90° β-turn every second residue, stabilized by three hydrogen bonds between the backbone amides. The first, second, fifth and sixth bases stack on one side of the monomer and pair with the corresponding complementary bases of a second monomer to form a dimer. The two remaining bases on each side of the resulting dimer form Watson-Crick pairs with the complementary bases of a second dimer, leading to a unique cage structure. The extra methylene groups in the homoalanyl residues enable stacking of the bases with an optimal distance between base-planes but also with an appreciable lateral displacement (slide).