Briefly described, embodiments of this disclosure include ligand-regulable transactivation systems, methods of producing ligand-regulable transactivation systems, methods of using ligand-regulable transactivation systems, reporter polynucleotides, method of producing reporter polynucleotides, activator fusion proteins, methods of producing activator fusion proteins, methods of regulating gene expression in vitro and in vivo for gene therapy, methods of screening estrogen receptor modulators with therapeutic treatments (e.g., anticancer, antiosteoporosis, and hormone replacement treatments), method of screening compounds (e.g., drugs and environmental pollutants) for the estrogenic effect, methods of evaluating the estrogen receptor pathway under different pathological conditions are provided, and the like.