The Internal Modifications click menu works by inserting the modification at the current end of the oligonucleotide you are building. If you need to go back and insert an internal modification after you have filled in your entire sequence, you will either need to:
1. Delete back to the position of the modification, select it in the menu and fill in your remaining sequence, or
2. Select the modification, then carefully cut and paste the modification into the correct position.
Please always be sure to double check your sequence before placing your order.