Of the first-generation oligonucleotide analogs, the class that has resulted in the broadest range of activities and about which the most is known is the phosphorothioate class. Phosphorothioate oligonucleotides were first synthesized in 1969 when a poly(rlrC) phosphoro-thioate was synthesized. This modification clearly achieves the objective of increased nuclease stability. In this class of oligonucleo-tides, one of the oxygen atoms in the phosphate group is replaced with a sulfur. The resulting compound is negatively charged, is chiral at each phosphorothioate phosphodi-ester, and is much more resistant than the parent phosphorothioate to nucleases (159).

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