Abstract: A method for enriching the GC base pair content of a DNA molecule the method comprising the steps of (a) providing a DNA template molecule in which at least some of the A residues are base paired with U residues and (b) replicating the DNA template molecule provided in step (a) under conditions in the replication reaction medium in which at least some of the U residues base pair with a G residue. Typically, the DNA template molecule in (a) is produced by replicating a first template DNA molecule in the presence of dUTP so that at least some of the T residues of the first template are replaced by U residues to form a second template molecule.