A tumor marker (also called a cancer biomarker) is one of the body's molecules that can be used to help detect and monitor the presence of certain types of benign (non-cancerous) and malignant (cancerous) tumors. An abnormally high level of a specific tumor marker in a laboratory test of tissue or a bodily fluid may suggest that a particular type of benign or malignant tumor is present in the body.
Purposes of laboratory tests involving tumor markers include:
• Screening for the presence of early-stage cancer
• Diagnosis of cancer (i.e., helping to distinguish between a non-cancerous and cancerous tumor).
• Selection of treatment (i.e., determining whether a patient is eligible for a specific type of therapy) of cancer
• Prognosis (i.e., determining the likely outcome) of cancer
• Monitoring the efficacy (i.e., effectiveness) of a particular therapy for cancer
• Monitoring whether early-stage cancer has recurred (i.e., re-occurred) following treatment
Examples of tumor markers include:
• Carcinoembryonic antigen (CEA), which may be elevated in patients with cancer of the breast, ovaries, lung, and gastrointestinal tract 
• CA 153, which may be elevated in patients with breast cancer. Sometimes levels of CA-153 are used to monitor treatment of breast cancer.
• CA-125, which may be elevated in women with ovarian cancer 
1. Dictionary. Accessed at www.cancer.gov.
2. Monitoring tumor markers. Accessed at www.chemotherapy.com.