A method for characterizing DNA, which comprises: (i) providing a population of DNA fragments, each fragment having cleavably attached thereto a mass label for identifying a feature of that fragment; (ii) separating the fragments on the basis of their length; (iii) cleaving each fragment in a mass spectrometer to release its mass label; and (iv) determining each mass label by mass spectrometry to relate the feature of each fragment to the length of the fragment.