The spacer phosphoramidites C3, 9, C12 and 18 are used to insert a spacer arm in an oligonucleotide. The compounds may be added in multiple additions when a longer spacer is required. 3'-Spacer C3 CPG may also act as a blocker of exonuclease and polymerase activity at the 3'-terminus. dSpacer is used to introduce a stable abasic site within an oligonucleotide. PC Spacer is a photocleavable C3 spacer modifier, part of our line of photocleavable (PC) modifiers.