A method for detecting sequence specific methylation in a biomolecule, comprising: (a) contacting the biomolecule with an S-adenosyl-L-methionine-dependent methyltransferase in the presence of a detectable cofactor of said methyltransferase; and (b) detecting whether the recognition sequence of said methyltransferase has been modified with the cofactor or a derivative thereof, wherein modification of the recognition sequence of said methyltransferase is indicative of an absence of methylation at said recognition sequence. Also disclosed is a cofactor specific for S-adenosyl-L-methionine-dependent methyltransferases, wherein said cofactor is an N-adenosylaziridine derivative with a reporter group attached to the 6 or 7 position of the adenine ring or attached to the aziridine ring. A complex of the cofactor and a methyltransferase a composition comprising the cofactor or the complex and the use of the cofactor or the complex for detecting sequence-specific methylation in DNA molecules are also disclosed.