integrate into a chromosome at random, either immediately or after one or two cleavage divisions. Because integration is a rare event, individual eggs rarely contain more than one integration site. If the transgene enters the germ line, it subsequently behaves as a Mendelian gene in meiosis.

Table 7-2 summarizes transgenic technologies commonly used in experimental mice, and those technologies are described in detail below.

Overexpression and Misexpression. A common use of transgenics is to overexpress a given gene either in the tissue where it is normally expressed or at ectopic sites (where it is not normally expressed). A transgene is constructed that includes the coding region of the gene and its own regulatory elements or those of another gene that will drive expression constitutively, inducibly, or ectopically. Some of the earliest transgenic experiments were ones in which transgenic mice were produced with a human-growth-hormone gene driven by the inducible metallothionein promoter. The use of a human gene provided a means of distinguishing the expression of the endogenous mouse-growth-hormone gene from the expression of the transgene.

Promoter and Enhancer Analysis. To identify the regulatory elements of a given gene, a series of transgenes can be constructed containing ever increasing amounts of the 5′ regulatory region linked to an unrelated gene with an assayable gene product (a reporter gene) or the gene itself. Then, a series of transgenic animals is produced with those constructs and assayed for the expression of the reporter gene or gene product. In this way, regulation of levels of expression and tissue specificity of expression can be assigned to specific positions in the regulatory region of the gene under study. This has been widely done, for example, in the analysis of the expression of the 39 Hox genes of the mouse.

Antisense Transgenes. Transgenes can be constructed to contain antisense sequences. If expression of an antisense transgene is directed to the tissues where the endogenous gene of complementary coding sequence is being expressed, antisense RNA transcribed from the transgene can hybridize to the endogenous mRNA and reduce its translation. This is useful for assessing the function of the endogenous protein.

Gene Trapping. A serendipitous means of finding new genes includes the phenomenon of insertional mutagenesis, where, by chance, a transgene inserts into a chromosomal gene causing a mutation that results in an unexpected mutant phenotype. A similar procedure was discussed above for P-element insertion in Drosophila. Another means of screening for new genes in the mouse is to make transgenics using a promoterless reporter gene. Successful expression of the reporter transgene will indicate its intergration near an endogenous promoter, and the expression pattern can provide information about the endogenous gene that

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