The pCA-ZtTA construct was designed with a CMV enhancer/chicken beta-actin core promoter (pCA), a loxP-flanked beta-geo (lacZ-neomycin phosphotransferase fusion) transcriptional STOP cassette, the tetracycline-controlled transactivator protein (tTA or "Tet-Off") open reading frame, and an SV40 T-antigen poly(A) signal. This pCA-ZtTA construct was subcloned into the pROSA26-PA targeting vector to generate the final targeting construct pROSA26-ZtTA. (J:161840)