Studies of the human genome have prompted the development of several cloning systems that can manipulate large fragments of human DNA as functional units. The discoveries in yeast of the sequences for replication (autonomously replicating sequence [ARS]), centromeres, and telomeres have allowed the creation of large linear molecules (yeast artificial chromosome [YAC]), which can replicate and segregate as chromosomes (1 ). YACs have been used to generate genomic libraries from different organisms with megabase insert size and have become an essential tool for physical and genetic mapping of various mammalian genomes (2 ). Nevertheless, the instabilities associated with YAC libraries (chimerism and deletions) have led to the development of alternative cloning systems (3 ).
