A phosphorothioate group is a modified phosphate group with one of the oxygen atoms replaced by a sulfur atom. In a phosphorothioated oligo (often called an "S-Oligo"), some or all of the internucleotide phosphate groups are replaced by phosphorothioate groups.

The modified "backbone" of an S-Oligo is resistant to the action of most exonucleases and endonucleases. Such S-Oligos may be useful in some "antisense" applications because of their enhanced stability.

Biolegio can produce phosphorothioate sites between all residues, or at specified locations within a sequence. A useful option is to sulfurize only the last few residues at each end of the oligo. This results in an oligo that is resistant to exonucleases, but has a natural DNA center.