Abstract: The invention relates to a novel method for altering the sequence of a nucleic acid molecule using repair recombination in a simple one component system. The frequency of the recombination reaction is high, allowing a range of feasible selection strategies to identify successful recombination events. The method involves the steps of bringing a first nucleic acid molecule into contact with a second nucleic acid molecule in the presence of a phage annealing protein into contact with a second nucleic acid molecule in the presence of a phage annealing protein, or a functional equivalent or fragment thereof, wherein said first nucleic acid molecule comprises at least two regions of shared sequence homology with the second nucleic acid molecule, under conditions suitable for repair recombination to occur between said first and second nucleic acid molecules; ad selecting a nucleic acid molecule whose sequence has been altered so as to include sequence from said second nucleic acid molecule.