Alpha-fetoprotein (AFP) is a major plasma protein produced by the yolk sac and the liver during fetal development. In adults, AFP expression is often associated with hepatoma or teratoma. However, hereditary persistance of AFP may also be found in individuals with no obvious pathology. AFP is thought to be the fetal counterpart of serum albumin, and the AFP and albumin genes are present in tandem in the same transcriptional orientation on chromosome 4. AFP is found in monomeric as well as dimeric and trimeric forms, and binds copper, nickel, fatty acids and bilirubin. AFP levels in amniotic fluid is used to measure renal loss of protein to screen for spina bifida and anencephaly.