The guidelines published by the European Society for Medical Oncology mention that useful tumor markers for diagnosing the primary tumor site include human chorionic gonadotropin (hCG) and α-fetoprotein (AFP) in patients with poorly differentiated carcinoma of midline distribution for germ-cell tumors, PSA in men with bone metastases for prostate cancer, cancer antigen 125 (CA125) in women with primary peritoneal serous adenocarcinoma for ovarian, fallopian tube, and peritoneal cancers, and thyroglobulin for differentiated thyroid cancer [1, 8–13]. The gene discussed is MUC16; the disease is peritoneum cancer.