While mutagenesis of single genes is now common practice in molecular biology, engineering multiple target genes still requires complex cloning techniques and thus is limited to expert laboratories. Here, we describe "Reiterative Recombination," a user-friendly DNA assembly technique in Saccharomyces cerevisiae for the integration of an indefinite number of DNA fragments sequentially into the yeast genome. The high efficiency of chromosomal integration can further be utilized for the assembly of large combinatorial libraries for metabolic engineering.