(The following is only one paragraph from this article - as time permits, I'll include more of the article. Most of the rest of the article includes other types of tumor markers.)

Alpha-fetoprotein (AFP) is a protein similar to albumin, the major protein in human blood. It is produced in the liver, yolk sac and gastrointestinal tract of the developing fetus, and small amounts of AFP are normally present in adult serum. Increased concentrations of AFP are found in most cases of liver cancer and germ cell tumors of the ovary and testis, as well as in about 10% of patients with other forms of cancer that have metastasized to the liver. Non-cancerous diseases of the liver, such as hepatitis and cirrhosis, are also associated with increased AFP levels. Since many different conditions can elevate the serum AFP concentration, its main use is in monitoring the response of liver and germ cell cancers to treatment.