APC: Gene named adenomatous polyposis coli, located on the q21 region of chromosome 5. This gene is found to be mutated in a hereditary disease called familial adenomatous polyposis (FAP); adenomatous polyps with high malignant potential cover the mucous membrane of the colon. Apoptosis: Also known as programmed cell death or cell suicide is a normal physiological phenomenon, which is an active process under genetic control. In this process, as opposed to necrosis, the cell shrinks and the DNA is digested by enzymes. The fragmented cellular particles are then eliminated by phagocytosis. Apoptotic index: Percentage of cells in a population destroyed by apoptosis. Arachidonate: The anion of an essential unsaturated fatty acid containing 20 carbon atoms and four double bonds. Humans use this fatty acid to synthesize regulatory molecules such as prostaglandins. It is present in fatty animal tissue and is also synthesized from dietary linoleic acid. Aromatase: An enzyme complex that catalyzes the conversion of testosterone to estradiol. Arterioles: Minute arterial branches, especially just proximal to capillaries. Ascites: Effusion and accumulation of fluid in the abdominal cavity. Ascorbates: Derivatives of ascorbic acid, Vitamin C. Ataxia-telangiectasia: A complex hereditary disorder that affects many systems such as nervous and immune system of an individual. There is an increased sensitivity to ionizing radiation caused by a defect in DNA repair. The affected person also shows an increased risk of cancer. Atherosclerosis: A very common form of arterial disease in which deposits of yellowish plaques (atheromas) containing cholesterol. Fat material is formed within the intima and inner media of large and medium-sized arteries. ATP: Adenosine triphosphate, where adenosine, a nucleoside is attached through its sugar (ribose) group to three phosphate groups. This molecule contains high energy phosphates. Atrophic gastritis: Chronic gastritis with atrophy of the mucous membrane and glands causing reduced secretion of gastric juice. Attenuated Virus: Virus whose virulence is reduced by some means. Atypical: Irregular type. Autoantibody: Antibody that reacts with an antigen that is a normal component of the body. Autoimmunity: A condition in which an individual's immune system starts reacting against his or her own tissues, causing diseases such as lupus. Autoimmune disease: Disease caused by the production of antibodies (special proteins produced by immune cells in response to antigens) against self-proteins. These antibodies destroy important proteins of the body and cause disease, e.g., Grave's disease when antibodies specific for thyroid stimulating hormone receptors are formed. Autoimmune disorder: Abnormality of function due to autoimmunity, a condition characterized by a specific humoral or cell-mediated immune response against constituents of the body's own tissues (self antigens or autoantigens). Autologous: Related to self, originating within an organism itself. Autosomal: Pertaining to an autosome, any ordinary paired chromosome except sex chromosomes. Avidin: A specific protein in egg albumin that interacts with biotin, a member of vitamin B group. Axillary node: Lymph nodes under the arm.