IDT's Azide modification uses an NHS Ester functional group to attach an azide moiety at the 5', 3' or any internal position in an oligo. This azide moiety may subsequently be used to attach alkyne modified groups through 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.