In this study, the ER-mediated activities of individual PCB congeners, which were identified as principal contaminants present in serum of human population in the studied area, were investigated using the T47D breast cancer cell line stably transfected with the luciferase reporter gene under control of estrogen-responsive elements, detecting the direct activation of ER (the ER-CALUX assay) (Legler et al. 1999). The gene discussed is ESR1; the disease is breast cancer.