We here present a novel covalently linked base pair via Schiff base formation between 5-formyluracil (U-f) and 5-aminocytosine (C-Am). Formation of the Schiff base linkage proceeds reversibly and does not require any additives. The cross-linked DNA is very stable under denaturing conditions, whereas it completely dissociates upon heating at 90 degrees C. The pairing ability of C-Am and U-f is very specific and is applicable to the detection of fU, which is the major oxidative lesion of T in DNA. We propose the Schiff base linkage as a new artificial base pairing scheme to create functional DNAs.