They mainly belong to three categories of tumor antigens: (i) tumor-associated antigens: antigens whose expression is enriched but not specific to cancer cells (e.g., AFP, GPC-3); (ii) cancer–testis antigens: antigens whose expression is limited to cancer cells and reproductive tissues but not adult somatic tissue (e.g., NY-ESO-1, MAGE); (iii) viral-derived cancer antigens: antigens expressed by cancer cells derived from an oncogenic viral origin (VHB, VHC). Here, AFP is linked to neoplasm.