Tagging neural ensembles using a c-fos promoter tTA/TetOLacZ-tTA mouse. The mouse has two transgenic alleles. One allele encodes the doxycycline sensitive tetracycline transactivator (tTA) under the control of the c-fos promoter. tTA binds the tetracycline operator (TetO) sequence to induce transcription of the downstream gene, and the induction activity is suppressed by the presence of doxycycline. The other expresses the secondary transactivator (tTA*) and tauLacZ under the control of TetO. tTA* is both insensitive to doxycycline and constitutively active, therefore, once expressed, tTA* activates TetO regardless of doxycycline. A. tTA is expressed exclusively in neurons where the c-fos promoter is activated. However, in the presence of doxycycline, tTA activity is inhibited and hence prevents interaction with TetO. B. After withdrawal of doxycycline, tTA is activated and binds to TetO to induce expression of tTA* and tauLacZ protein in c-fos activated neurons (purple). C. After re-administration of doxycycline, tTA becomes inactivated once again but as tTA* is still present, it can maintain the expression of tauLacZ by activating TetO in the positive-feedback loop. This system thereby allows the permanent tagging of neurons activated even in the absence of doxycycline.