Alpha-fetoprotein (AFP) is a serum glycoprotein protein produced in the liver or yolk sac of fetal staged mammals. AFP synthesis is minimal after birth and trace amount is expressed in the adult liver. AFP gene expression is regulated by the interactions between steroid hormone receptors and transcriptional factors in separate signal transduction pathways. AFP functions as a binding and transporting ligand and cell growth regulator. An elevated expression level of AFP has been implicated in colorectal, ovarian, pancreatic, testicular, and certain liver cancers. High level of AFP is also seen some diseases, such as hepatitis and colitis. AFP is used as a screening marker for fetal abnormalities in pregnant women, such as Down syndrome. Recently, AFP has been introduced as an anti-cancer drug-ligand carrier, transporting drugs to target tumor cells, increasing anti-tumor efficiency.