The invention relates to improved methods for the modification, including recombination, of genes in cells. More specifically, the invention relates to the increased efficiency of modification, including recombination, by introduction of a DNA-modifying molecule into a cell cycle synchronized cell. Additionally, the invention relates to target DNA that has been modified, mutated or marked by the approaches disclosed herein. The invention also relates to cells, tissue, and organisms which have been modified by the invention's methods.