Target Discovery and Validation Reviews and Protocols

182 Houdebine
Fig.7. Example of a vector expressing a foreign gene under the control of tetracycline
or derivatives with a low background of expression.
Fig. 8. Vector for gene trapping. The random integration of the vector is performed
in ES cells and further used to develop embryos. The expression of β-galactosidase-neo r
genes reveals that the vector was integrated into a gene. The inhibition of the interrupted
gene may be correlated with a phenotypic effect.
retains a slight basal activity in the absence of the inducer. To reduce the background
expression, factors acting as enhancers and silencers can be alternatively
expressed under the action of an inducer such as tetracycline. Such a system constructed
in our laboratory is described in Fig. 7. Transgene expression also can
be controlled at the translation level (106).
3.6. Vectors for Gene Trap
The systematic identification of genes by genome sequencing and the establishment
of gene expression pattern by DNA chips may be insufficient to determine
the role of a gene in a given biological function. Gene trapping is currently
being used to identify genes of interest. For this purpose, vectors containing a
marker gene, an intron splicing site, and a transcription terminator, but devoid
of promoter, are introduced by electroporation in ES cells (Fig. 8). The cells are