Animal studies have shown stem-like properties of adult hepatocytes and stem-like properties of biliary epithelial cells with the latter originating form the canaliculi of Hering. Increase of serum alpha-fetoprotein is associated with proliferative cell activity of these populations. The extent of AFP synthesis depended largely on animal species and their respective strains. Hepatocyte regeneration may be blocked owing to the nature of the damage. Then, liver repair follows pathways wich involve biliary epithelial cells (oval cells). Oval cells and their progenies are descendents from the canaliculi of Hering and small intrahepatic bile ductules. They act as stem-like cells, they reach levels with fetal gene activation during proliferative stages and AFP synthesis as a signal of reversal ontogeny. AFP resurgence in liver cancer reflects traits of retro-differentiation or stages of maturation arrest during cell differentiation. AFP re-expression is not confined to malignancy because such molecules can also resurge during non-malignant growth. The term “transitory cell antigens” is an appropriate naming.