In this regard, serum sEGFR concentrations also have been correlated with the female gonadotropin follicle stimulating hormone, as well as with responsiveness to treatment with the aromatase inhibitor letrozole in breast cancer patients [32,33], suggesting a complex relationship between this novel serum biomarker and steroid hormones such as estrogen and progesterone. This evidence concerns the gene CYP19A1 and breast carcinoma.