Immunohistochemically, the cyst’s epithelium was immunoreactive to cytokeratin 7 (CK-7), epithelial membrane antigen (EMA) and Wilms tumor 1 (WT1), and it was immunonegative for the progesterone receptor (PR) and cytokeratin 20 (CK-20), while only the stromal cells were positive for estrogen receptors (ER), therefore, a retroperitoneal Müllerian cyst was diagnosed. The gene discussed is MUC1; the disease is cyst.