Compositions and methods for detecting 5-methylcytosine in a nucleic acid are disclosed. A 5-methylcytosine discriminator, which is a deoxyribonucleosidetriphosphate comprising a cytosine-pairing moiety such as a guanosine and a moiety which hinders hydrogen bonding between the cytosine-pairing moiety and a 5-methylcytosine is described. The discriminator is able to base pair with a cytosine but not a 5-methylcytosine. A 5-methylcytosine comprised by a target nucleotide can be detected in a reaction using a DNA polymerase and a primer hybridized immediately adjacent to the target nucleotide. In the reaction, pyrophosphate released upon incorporation of a dNTP complementary to a target nucleotide is detected. Lack of incorporation of the discriminator, but incorporation of a dGTP, can indicate that the target nucleotide is a 5-methylcytosine.