Materials and Methods Nucleosides were obtained from Pharma-Waldhof (Düsseldorf, West Germany). Benzoyl chloride, anisoyl chloride, and isopropionyl chloride were purchased from EGA Chemie (Steinheim, West Germany) and were redistilled before use. IFf-Tetrazole was a product of EGA Chemie and was purified by sublimation at 100 C and 0.05 mmHg prior to use. Pyridine, 2, 6-lutidine, and iV-ethyldiisopropylamine were purchased from Merck (Darmstadt, West Germany) and were refluxed with and then distilled from calcium hydride and stored over 4-Á molecular sieves. Acetonitrile used in the solid-phase nucleotide synthesis was an HPLC grade reagent from J. T. Baker Chemicals (Deventer, Holland). It usually contains 0.01% water but was stored over 4-Á molecular sieves and otherwise used as supplied. THF1 used in the solid-phase synthesis was Merck driedreagent (maximum H2G content 0.01%) as was Me2SO (maximum H2G content 0.03%) used to prepare phosphorothioate-containing dimers. These solvents were also stored over 4-Á molecular sieves. All other solvents used in the preparation of oligonucleotides were pa grade and were stored over 4-Á molecular sieves. Nuclease PI (200 units/mg) was obtained from Sigma (Munich, West Germany), and alkaline phosphatase (fromcalf intestine, 2500 units/mg, molecular biologygrade) was purchased from Boehringer Mannheim (West Germany). Polynucleotide kinase (from T4 infected E. coli, 5 units/jitL) was a product