Alpha-fetoprotein (AFP) is a plasma protein produced by the liver and the yolk sac during fetal development. AFP is believed to be the fetal analog of serum albumin and it acts as a carrier protein for steroids, bilirubin, fatty acids, and retinoids. In addition to its function as a transport protein, AFP can exert immunosuppressive activity, regulate cell proliferation and apoptosis, initiate intracellular signaling, and contribute to cell invasion. Low levels of AFP in the maternal serum are associated with a higher incidence of Down syndrome, while higher levels of AFP are an indicator of spina bifida and anencephaly. Because of this, maternal AFP serum level is commonly used to screen for Down syndrome and neural tube defects. Furthermore, some tumor types have been found to secrete AFP including hepatoblastoma and hepatocellular carcinoma. Thus, AFP can be used as a tumor marker to monitor a patient's response to a treatment such as chemotherapy.