The introduction of yeast artificial chromosomes (YACs) as cloning vectors in 1987 has significantly advanced the analysis of complex genomes (1 ). The capability of cloning large DNA (100–2000 kb) as YACs has accelerated the construction of physical maps and contig building (a contiguous set of overlapping clones). YAC contigs now cover entire human chromosomes (i.e., Y and 21) (2 ,3 ) and small genomes (i.e.,Schizo-saccharomyces pombe) (4 ), and large YAC contigs cover much of the human genome (5 ). The main advantages of YACs over prokaryotic-based cloning systems are their large insert capacity and ability to maintain sequences that are unstable or not well represented in bacteriophage or cosmid genomic libraries (6 ). Therefore, YACs complement existing cloning vectors (cosmids, bacteriophage) and new cloning vectors (PI bacteriophage [PI], bacterial artificial chromosomes [BACs], and PI-derived artificial chromosomes [PACs]; for review,seeref.7 ) in mapping and chromosome walking projects (6 ,8 ).
