The glucocorticoid receptor belongs to the nuclear receptor superfamily which represents one of the most abundant and important classes of transcriptional regulators. Glucocorticoid receptor (GR) signaling is critically involved in different aspects of metabolic homeostasis especially under conditions of physiological stress. The immunosuppressive activity of glucocorticoids is widely used to treat inflammatory and autoimmune diseases, but might be also of interest for the treatment of certain cancers. GR shares a common domain structure with other members of the nuclear hormone receptor superfamily. The N-terminal A/B domain contains transactivation activity. It is followed by the highly conserved DNA-binding domain (C-domain) which is connected by a flexible hinge region (D domain), which includes the nuclear localization signal, to the C terminal ligand binding domain (LBD, E domain). The LBD combines a number of different functions including ligand binding, receptor dimerization, transactivation/repression and binding sites for numerous cofactors. In its inactive state GR is bound to a cytosolic protein complex including hsp90 and different inhibitory proteins, which prevent GR from translocation into the nucleus. Upon binding of its cognate ligand, GR undergoes a conformational change which leads to the dissociation of the cytosolic protein complex and the exposure of a nuclear localization signal, thus allowing the GR-ligand complex to translocate into the nucleus, where it mediates positive and negative gene regulatory effects by binding to GR specific DNA response element and association with a variety of cofactors. Transactivation requires the binding of GR to specific palindromic sequences in the regulatory regions of target genes called glucocorticoid response elements (GRE). The mouse mammary tumor virus (MMTV) long terminal repeat used for the development of this cell lines contains several GRE’s.