The ability to propagate long fragments of DNA cloned in Saccharomyces cerevisiae as yeast artificial chromosomes (YACs) represents a significant advance in cloning technology. Its introduction in 1987 (1 ) demonstrated the cloning of human DNA with the pYAC series of vectors in the yeast host AB1380. Subsequently, many other libraries have been generated from different DNA sources, with varying average insert sizes and using different combinations of YAC vectors and yeast hosts (2 –5 ). Because DNA replication initiates only once/cell cycle in yeast cells at each chromosomal origin of replication (6 ) with equal segregation of replication products (7 –9 ), these clones consist of only one YAC/haploid cell. This limits the sensitivity and convenience of using YACs.