Substrates are designed to examine the effect of altering the CpG site context on hTDG activity for the excision of T, U, FU, ClU, and BrU. For the YpG·X substrates, the 5′-flanking pair is varied for G·X substrates. For example, TpG·X indicates that the G·X target pair has a 5′-flanking T·A pair. For the CpA·X substrates, the target base is paired with A rather than G, while the 5′ C·G pair is retained. For the YpA·X substrates, the target base is paired with A and the 5′ flanking base pair is varied.