REPORTER GENES IN TRANSGENIC MICE

Although in vivo models utilizing endogenous reporter genes have been exploited for many years, the use of reporter transgenes to dissect bi ological issues in transgenic animals has been a relatively recent dev elopment. These transgenes are often, but not always, of prokaryotic o rigin and encode products not normally associated with eukaryotic cell s and tissues. Some encode enzymes whose activities are detected in ce ll and tissue homogenates, whereas others encode products that can be detected in situ at the single cell level. Reporter genes have been us ed to identify regulatory elements that are important for tissue-speci fic gene expression or for development; they have been used to produce in vivo models of cancer; they have been employed for the study of in vivo mutagenesis; and they have been used as a tool in lineage analys is and for marking cells in transplanation experiments. The most commo nly used in situ reporter gene is lacZ, which encodes a bacterial beta -galactosidase, a sensitive histochemical marker. Although it has been used with striking success in cultured cells and in transgenic mouse embryos, its postnatal in vivo expression has been unreliable and disa ppointing. Nevertheless, the ability to express reporter genes in tran sgenic mice has been an invaluable resource, providing insights into i n vivo biological mechanisms. The development of new in vivo models, s uch as those in which expression of transgenes can be activated or rep ressed, should produce transgenic animal systems that extend our capac ity to address heretofore unresolved biological questions.