Capping
-A small percentage (about 2%) of support-bound nucleotide will fail to undergo addition.
-These truncated, or failure sequences, will remain attached to the support.
-They can propagate in subsequent coupling steps producing a sequence with one less base.
-Capping the remaining free hydroxyls by acetylation eliminates this problem.
-Capped failure sequences are prevented from participating in the rest of the synthesis reactions.