Terms Starting with Letter: A Adenine (A) One of the bases found in nucleic acids. See Base pair. adenovirus A type of virus that can be genetically modified for delivering therapeutic DNA during gene therapy. affinity Chemical attraction or force between molecules causing them to remain in a combined state. allele The alternate form of a gene at a corresponding locus on homologous chromosomes. amino acid Molecules of the form NH2CHRCOOH that can be linked through peptide bonds forming proteins. amplicon Multiple copies of short DNA sequences resulting from PCR amplification. amplification Molecular biology: an increase in the number of copies of a specific DNA fragment. Molecular imaging: an increase in imageable signal. angiogenesis The growth of new blood vessels (needed for tumor growth). Annexin V A protein that, when radiolabeled, can be used for apoptosis imaging in mice because of it's high affinity to membrane-bound phosphatidylserine (an apoptosis marker). annihilation An interaction between a particle and its antiparticle in which the mass of the particles are converted to electromagnetic energy in the resultant form of photons. Two equal energy 511 keV photons are produced when a positron and electron combine and their mass is converted into energy. The annihilation photons are emitted 180 degrees to one another. antibody Any of various proteins in the blood that are generated in reaction to foreign proteins or polysaccharides, neutralize them and so produce an immunity against certain microorganisms or their toxins. Antibodies are commonly used in biotechnology and imaging because of this specific binding properties.(Synonym: immunoglobulin.) antigen A substance that when introducd into the body, stimulates the production of an antibody. antisense probe Labeled either radioactively or immunologically, probes are single-stranded DNA or RNA molecules of specific base sequence used to detect the complementary base sequence by hybridization. antisense RNA RNA molecule that inhibits translation of a particular mRNA by hybridization to a complementary sequence. antisense therapy Treatment of a disease through the administration of antisense RNA designed to bind in a sequence-specific manner to an mRNA and thus to interfere with the expression of a disease-correlated gene. antiserum Blood serum from an animal immunized against a specific antigen and therefore containing antibody to that antigen. apoptosis Genetically determined destruction of cells from within due to activation of a stimulus or removal of a suppressing agent or stimulus that is thought to exist to explain the orderly elimination of superfluous cells. Also called programmed cell death. attenuation In PET, attenuation refers to a loss of energy in the gamma-ray signal due to absorption, scattering, divergence, or other causes as the gamma-ray travels through the tissues of the body. The degree of attenuation depends on the medium through which the gamma-ray travels. For example, bone causes higher attenuation than does soft tissue. Attenuation correction can be applied to correct this weakening of the signal in a quantitative manner using PET. This is in contrast to SPECT where attenuation correction is both difficult and approximate. autoradiography A technique in which radiolabeled compounds are used to study the cellular distribution of macromolecules or the physiologic processes in which these molecules are involved. In this technique, labelled tissue or cells are overlaid with a photographic emulsion that is sensitive to radiation. Development of this emulsion reveals the distribution of the radiolabeled molecules. avidin-biotin system Two molecules interacting with the strongest affinity known in biology (effectively irreversible binding). The avidin-biotin system is used in molecular biology for linking a targeting molecule to a label or for in vivo pretargeting in imaging. axial Makes up the main axis of the body: the head, neck, and trunk. axillary armpit region