1) A protein normally produced by a fetus. AFP levels are usually undetectable in the blood of healthy adult men or women (who are not pregnant). An elevated level of AFP suggests the presence of either a primary liver cancer or germ cell tumor.

2) A fetal blood protein present abnormally in adults with some forms of cancer (as of the liver) and normally in the amniotic fluid of pregnant women but with very low levels tending to be associated with Down syndrome in the fetus and very high levels with neural tube defects (as spina bifida) in which the tube remains open.