For example, patients with known ER positive breast cancer in the past who now present (1) with lesions on CT, MRI, [18F]-FDG-PET, bone scintigraphy, etc., in which still a differential diagnosis exists (Figure 1), (2) with a new different primary tumor and lesions that could be metastases of the earlier breast cancer or of the new tumor, (3) with a solitary lesion of which biopsy is not feasible due to location and knowledge of the ER status is important for therapy decision making, etc. Here, ESR1 is linked to breast cancer.