Considering the inconsistent results of previous studies—specifically clinical correlations and the prognostic value of ER expression in lung cancers—its function may vary due to variations in patient gender (male vs. female), location of ER expression (nucleus vs. cytoplasm), ER isoforms (ERα vs. ERβ), and epitopes that each anti-ER monoclonal antibody can recognize. This evidence concerns the gene ESR1 and lung cancer.