Hydrolyzes deoxynucleoside triphosphates (dNTPs) to the corresponding nucleoside monophosphates. Has a strong preference for modified dCTP. Activity is highest with 5-iodo-dCTP, followed by 5-bromo-dCTP, unmodified dCTP, 5-methyl-dCTP and 5-chloro-dCTP. Hydrolyzes 2-chloro-dATP and 2-hydroxy-dATP with lower efficiency, and has even lower activity with unmodified dATP, dTTP and dUTP (in vitro). Does not hydrolyze ATP, UTP, ITP, GTP, dADP, dCDP or dGTP. May protect DNA or RNA against the incorporation of non-canonical nucleotide triphosphates. May protect cells against inappropriate methylation of CpG islands by DNA methyltransferases.