Immunohistochemistry was positive for calretinin and negative for carcinoembryonic antigen (CEA), Wilms' tumor 1 antigen (WT-1), estrogen receptor (ER), progesterone receptor (PR), and CD-15; these findings were consistent with reactive mesothelial cells as well as peritoneal mesothelioma. Here, ESR1 is linked to peritoneal mesothelioma.