A transgene containing a bidirectional tetracycline promoter (tetO) driving expression of human lacZ (5' of tetO) and GR1 (3' of tetO) was inserted into the locus by homologous recombination. Selection was achieved by restoring the locus that is defective in the ES cells used to generate this allele. Following activation of tetO, both GR1 and lacZ expression is detected. (J:157969)