Commentary YAC transgenes : Bigger is probably better

A number of recent articles have reported the successful generation of transgenic mouse lines carrying large fragments of DNA contained in yeast artificial chromosomes (YACs) (1-5). The ability to transfer such large segments of DNA provides a number of opportunities, including the analysis ofthe function of large genes or gene clusters in their natural DNA configuration, the study of the influence of normally remote control elements on gene expression, and the analysis of long-range effects of regulatory elements such as those involved in X chromosome inactivation and imprinting. Other applications of YAC transgenes include the study oflarge DNA fragments for their ability to complement or rescue the loss of function associated with genetic defects in mice, and thereby confirm the identity or nature of genes isolated by positional cloning (reviewed in