The Yeast Artificial Chromosome (YAC) (1 ) technology allows the cloning of large fragments of DNA (several hundreds of kilobase pairs, kbp) into the budding yeast Saccharomyces cerevisiae , and therefore bridges a gap between conventional cloning in bacteria (λ and cosmid cloning) and somatic cell hybridization techniques. This powerful cloning system has been used successfully for the construction of representative human (2 –5 ), mouse (5 –5 ), Drosophila (8 ), and Caenorrhabditis elegans (9 ) genomic libraries.