In Tet-Off Systems, the basal state is maintained in the presence of doxycycline (Dox), and induced by its withdrawal. Tet-On Systems are activated in the presence of Dox. System induction produces high-level transcription of Medaka polycystic kidney disease 2 (Pkd2) delta C-EGFP. Transgene: Medaka Pkd2 gene, EGFP, pTRE2hyg.