N -Methyladenine (m A) is the most abundant internal modification on mammalian mRNA. Very recently, m A has been reported as a potentially important ‘epigenetic’ mark in eukaryotes. Until now, site-specific detection of m A is technically very challenging. Here, we first reveal that m A significantly hinders DNA- and RNA-directed DNA synthesis. Systematic investigations of 5′-triphosphates of a variety of 5-substituted 2′-deoxyuridine analogs in primer extension have been performed.