ESR1 and cancer: Several currently available circulating biomarkers, such as carbohydrate antigen 19-9 (CA 19-9), carcinoembryonic antigen (CEA), pepsinogen (PG) I/II ratio, estrogen receptor (ER), and progesterone receptor (PR), are being used as non-invasive methods for cancer diagnosis without involving a biopsy or a surgical procedure [11]–[13]; Unfortunately, they also suffer from the limitation of low sensitivities and specificities.