GeneBLAzer®ER alpha DA(Division Arrested) cells and ER alpha-UAS-bla GripTite™ cells contain the ligand-binding domain (LBD) of the human Estrogen receptor alpha (ER alpha) fused to the DNA-binding domain of GAL4 stably integrated in the GeneBLAzer®UAS-bla GripTite™ cell line. GeneBLAzer®UAS-bla GripTite™ cells stably express a beta-lactamase reporter gene under the transcriptional control of an upstream activator sequence (UAS). When an agonist binds to the LBD of the GAL4 (DBD)-ER alpha (LBD) fusion protein, the protein binds to the UAS, resulting in expression of beta-lactamase. Division Arrested (DA) cells are available in two configurations- an Assay Kit (which includes cells and sufficient substrate to analyze 1 x 384-well plate), and a tube of cells sufficient to analyze 10 x 384-well plates. DA cells are irreversibly division arrested using a low-dose treatment of Mitomycin-C, and have no apparent toxicity or change in cellular signal transduction. Both ER alpha DA cells and ER alpha-UAS-bla GripTite™cells are functionally validated for Z' and EC50 concentrations of 17-beta Estradiol. In addition, ER alpha-UAS-bla GripTite™cells have been tested for assay performance under variable conditions, including DMSO concentration, cell number, stimulation time, and substrate loading time (data available upon request). Additional testing data using alternate stimuli are also available.