Biotin (Azide) is attached to the oligo sequence through a 1,2,3-triazole linkage. The 1,2,3-triazole linkage is formed from the reaction of azides and alkynes in the presence of copper (I), in the click reaction. The internal version of this modification is attached to the oligo through a dT base. Incorporation of the internal version will add a dT nucleotide at that position. To avoid adding an extra nucleotide, replace an existing T nucleotide in your sequence with the required modification.