MUC5AC and carcinoma: Myxoid/mucinous changes in endometriosis are rare, but they are well described in decidualized cesarean scars.11 Some cases may grossly and microscopically resemble a pseudomyxoma peritonei, with pools of acellular mucin involving peritoneal surfaces.12 In small biopsies, this presentation of endometriosis may be difficult to distinguish from pseudomyxoma peritonei with its mucin-rich background and signet ring carcinoma cells.