A collection of powerful new techniques for oligonucleotide synthesis and for the use of modified oligonucleotides in biotechnology. Among the protocol highlights are a novel two-step process that yields a high purity, less costly, DNA, the synthesis of phosphorothioates using new sulfur transfer agents, the synthesis of LNA, peptide conjugation methods to improve cellular delivery and cell-specific targeting, and triple helix formation. The applications include using molecular beacons to monitor the PCR amplification process, nuclease footprinting to study the sequence-selective binding of small molecules of DNA, nucleic acid libraries, and the use of small interference RNA (siRNA) as an inhibitor of gene expression.