Since their mtroductlon in 1987, yeast artificial chromosomes (commonly referred to as YACs) have had a tremendous impact on the field of molecular genetics The ability of YACs to carry and propagate DNA fragments of up to -2 mllhon base pairs in size has been instrumental in the construction of the first- and second-generation physical maps of the human genome (1,2) YACs have assisted in the generation of detailed maps of discrete chromosomal regions and slgmficantly speeded up posltlonal cloning projects leading to the ldentlficatlon and cloning of a variety of genes involved in inherited diseases (3,4) YACs have also been used for studies on gene function via complementatlon of mutations as well as for the transfer of complete genomlc gene sequences for the generation of transgemc animals Widespread use of this broad range of appllcatlons has been facilitated by the avallabrhty of a number of high-quality genomlc YAC libraries via centralized screening centers This clearly illustrates the scientific advances that can be made when tools and technology are shared in the spirit of cooperation.