The transgene consists of the tet operator driving expression of a bicistronic transcript encoding activated rat Erbb2 (Neu), having the Val664Leu missense mutation, and firefly luciferase, the latter translated from an internal ribosome binding sequence (IRES) upstream of its coding sequence. In a mouse bitransgenic for this transgene and a tetracycline transactivator transgene, administration of doxycycline reversibly induces expression of Erbb2 and luciferase in tissues where the transactivator is expressed.