Terms Starting with Letter: G gadolinium Contrast agent used in MR imaging that has a high magnetic moment changing the relaxivity of protons and therefore the MR signal. Due to toxicity, it is usually chelated. gamma-ray The energy released in a nuclear transformation to a more stable state may appear as a photon of electromagnetic radiation. Photons of nuclear origin are called gamma-rays. ganciclovir (GCV) Commonly used for treating active cytomegalovirus (CMV) infection. Labeled gancyclovir is used as a PET reporter probe. gastrin-releasing peptide receptor (GRP receptor) Often overexpressed receptor in proliferating cancer cells making it a possible target for imaging contrast agents. gene Physical and functional unit of heredity consisting of the entire DNA sequence needed for the synthesis of a functional polypeptide or RNA molecule. gene arraying, gene chip See microarray technology. gene expression Process by which the information encoded in a gene is converted to product, usually protein. gene mapping Determination of the relative positions of and distance between genes on a chromosome. gene product Product of gene expression (RNA or protein). The amount of gene product is used to measure how active a gene is. Abnormal amounts can then help locate specific alleles. gene targeting Process by which a therapeutic gene and/or a reporter gene is directed to a pathologic structure (i.e.: tumor). gene therapy Treatment or prevention of disease through transfer of normal genes or therapeutic genes to correct a genetic defect. gene transfer Transfer of transgenic material into a host cell by one of the following ways: transduction, transfection or transformation. genetic code Code by which nucleotide triplets (codons) in RNA specify amino acids in proteins. genetic engineering The manipulation of genes using molecular biology methods to produce proteins with modified function. genetic marker Segment of DNA (either a gene or other section that has no known function) whose inheritance can be followed. genetics The study of heredity. genome Total genetic information carried by a cell or organism. genomics The science of mapping and sequencing the entire genome of an organism. genotype The complete genetic constitution of a cell or organism. glioblastoma A highly malignant brain tumor (grade IV astrocytoma). These neoplasms grow rapidly and invade extensively. glycolysis The metabolic pathway from glucose to pyruvate with a net production of 2 molecules of ATP per molecule of glucose. Pyruvate can then be converted to lactate (anaerobic glycolysis) or enter the tricarboxylic acid cycle (TCA cycle). Complete oxidation of glucose through glycolysis and the TCA cycle yields a net production of 38 molecules of ATP, 6 molecules of CO2 and 6 molecules of H2O per molecule of glucose. The pathway of glycolysis and oxidation in the TCA cycle is referred to as aerobic glycolysis. glycolytic Refers to glycolosis. graft copolymer Two polymer types grafted onto each other used as blood pool agents or delivery vehicles. green fluorescent protein (GFP) Protein found in jellyfish that fluoresces at a wavelength of 395 nm. The isolated and cloned GFP gene is used as a reporter gene. guanine (G) One of the bases found in nucleic acids. See Base pair.