Figure 1: Schematic representation of the process of creating transgenic mouse mammary adenocarcinoma cells (locus derepression assay cells or LDR) [29]. A construct containing the GFP gene linked to the estrogen receptor ligand-binding domain, driven by the CMV promoter and a neomycin resistance marker, was stably transfected into cells. GFP negative cells, stably expressing the neomycin resistant gene, were selected. With the addition of an epigenetic modulator, such as an HDAC inhibitor or a DNA methyltransferase inhibitor, the cells express GFP and can be visualized by fluorescence microscopy. When cells are then treated with estradiol, the GFP fusion protein translocates to the nucleus.