The best-characterized human tumor-associated antigens are the oncofetal antigens. CEA is a glycoprotein and member of the immunoglobulin gene superfamily and is elevated in colorectal cancer. a-Fetoprotein (AFP) is analogous to albumin and elevated in hepatocellular carcinoma. Prostate-specific antigen (PSA) is elevated in prostatic cancer. CEA, AFP, and PSA are all glycoproteins. Melena refers to altered (black) blood per rectum, indicative of an upper gastrointestinal bleed. A patient's tumor markers are best used clinically in the monitoring of the efficiency of the antitumor therapy and remission periods posttreatment.