The title oligonucleotide and many related dodecamers have been extensively studied alone and as DNA-drug complexes, In practically all cases they were found to crystallize in the same space group, stabilized by interactions among the terminal guanine bases. Here me report new packing interactions (R3) in the presence of Ca2+. The oligonucleotides interact by placing their terminal guanines in the narrow groove of a neighbor molecule, an interaction which had never been found in dodecamers. (C) 1998 Federation of European Biochemical Societies.