CA125 (Cancer Antigen 125) isa protein in the blood used as a tumor marker, primarily for monitoring ovarian cancer, but it's not definitive for diagnosis as it's also elevated in many benign conditions like endometriosis, fibroids, and menstruation. A blood test measures CA125 levels, which helps doctors track treatment effectiveness, check for recurrence, or investigate pelvic lumps, especially in postmenopausal women where it's more specific. Normal levels are generally below 35 U/mL, but high levels do not automatically mean cancer, requiring further evaluation with imaging.