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IDT uses a simple notation for phosphorothioate bonds (PS bonds). Insert an asterisk (*) between the bases within your sequence to denote a phosphorothioate chemical modification.
The following example has a PS bond between the T and G bases: 5'- TTTTTT*GGGGGG-3'. PS bonds introduced between the last nucleotides at the 5'- or 3'-end of the oligo inhibit exonuclease degradation.
Order PS-modified oligos at: idtdna.com/site/order/oligoentry.