Structural modifications of nucleosides in which the 3'-or 5'- oxygen has been replaced with sulfur have been important tools in probing mechanistic and structural aspects of nucleic acid chemistry. They also show promise as potential antisense agents, as well as providing stabilization in thio-modified DNA duplexes with complementary RNA. The Cossticklabs have reported several methods for the automated incorporation of3'-S-phosphorothioates into oligonucleotides.