Methylation of thymine residues during oligonucleotide synthesis.

Thymine residues in an oligodeoxyribonucleotide are subject to methylation at N3 by the internucleotide methyl phosphotriester linkages. This alkylation occurs most rapidly in the presence of a strong base such as DBU, but also takes place, at a much slower rate, during oligonucleotide synthesis.