Yeast artificial chromosomes (YACs; 1 ) are rapidly becoming the major cloning system to study eukaryotic genomes by physical mapping and chromosome walking projects (1 –6 ). The main advantages of YACs over prokaryotic-based cloning systems are their large insert capacity (100–1500 kb), and their ability to maintain sequences that are unstable or not well represented in bacteriophage or cosmid genomic libraries.
