Spacers impart a number of desirable characteristics, such as stability to enzymatic degradation. Multiple spacers can be placed end on end to create extremely long spacers. They can also be placed between other oligonucleotide modifications. Even though these spacers are shown on the 5' terminus of the oligonucleotide, they can be placed internally as well. The C3 (Propyl) Spacer can also be placed at the 3' terminus and is an effective terminator.