We assessed the intratumor protein and mRNA expression levels of 5 genes related to estrogen synthesis and function–genes encoding the steroidogenic enzymes AKR1C3 and aromatase; ERα; and 2 estradiol/ERα target genes, BRCA1 and PR (Figure 1) – and determined their ability to predict responsiveness to AI therapy in postmenopausal women with recurrent breast cancer who failed to respond to adjuvant tamoxifen therapy. The gene discussed is ESR1; the disease is breast carcinoma.