Phosphates can be placed on the 3' or 5' termini of oligonucleotides. A 3' terminal phosphate can be used to block elongation although other modifications, such as 3' Propyl or a 2',3'-Dideoxy modified base, are more effective terminators. A 5' terminal phosphate is needed for enzymatic ligation of two oligonucleotides to form longer oligonucleotides.