Use of an internal ribosome entry site for bicistronic expression of Cre recombinase or rtTA transactivator

Conditional gene targeting depends on tissue and time specificity of recomb ination events. Endogenous promoters are often used to drive various transg enic constructs. To avoid the problems associated with reconstituting a spe cific expression pattern in transgenic animals by this method, we tested th e internal ribosome entry site of the encephalomyocarditis virus, to enable linkage of the Cre recombinase or rtTA transactivator to 3' untranslated e nds of endogenous genes. Here we report that these constructs function effe ctively in COS cells. The data suggest that these cassettes will be appropr iate for 3' targeting of mouse genes.