ESR1 and breast cancer: However, in primary gastric adenocarcinoma, ER and PR can be positively expressed in 20-28% of patients.19 In a few cases, metastatic breast cancer is negative for ER and PR, so a diagnosis cannot be made based on these two investigations alone.59 ER and PR can be used as markers; however, they are not always suitable diagnostic markers to confirm if a tumor has originated.11 Treatment of gastrointestinal metastases from breast cancer is discussed frequently in the literature.