To avoid polymerase extension at the 3' terminus, 2',3'-dideoxynucleoside and 3'-deoxynucleoside CPGs have proved to be effective. 2',3'- Phosphoramidites are designed to be used with the 5'-phosphoramidites and supports. Since these phosphoramidites have no DMT group, they are not compatible with purification by the DMT-on technique. Ion exchange HPLC or PAGE should be used to purify these dideoxy terminated oligos to ensure that shorter sequences (containing 3'-OH) groups are removed. (3'-Termination can also be effected using a 3'-3' linkage formed using 5'-supports, or 3'-spacer C3 CPG.)