To evaluate whether leptin signaling may have a role in driving aromatase inhibitor resistance, as an adequate aromatase/estrogen receptor (ER) α positive model system to study aromatase inhibitor response, we generated a therapy refractory cancer cell line by culturing parental human MCF-7 breast cancer cells stably transfected with an aromatase expression vector (MCF-7 aro) [38] continuously in the presence of the non-steroidal AI anastrozole (termed as AnaR cells). The gene discussed is ESR1; the disease is breast cancer.