Most of our modification are available for internal, and both external end. You can choose the side that your application recommends. The coupling possibilities for each modification are indicated in our price list.
If your assay procedure does not otherwise required, Bio-Synthesis recommends to attach modification to the 5'-end of your oligonucleotides (e.g. most PCR applications are still possible if the modification is attached tot he 5'-end).
In contrast, modifications at the 3'-end usually block this end for further enzymatic reactions (nevertheless, sensitive applications might detect elongation even of 3'-modified oligonucleotides. For complete blocking, we recommend inverted end or C3-Spacer modifications).