Radiodermatitis: A skin inflammatory reaction occurring as a result of exposure to biologically effective level of ionizing radiation. Radioimmunotherapy: Radiotherapy where cytotoxic radionuclides are linked to antibodies in order to deliver toxins directly to tumor targets. Therapy with targeted radiation rather than antibody-targeted toxins (immunotoxins). Radioisotope: Form of a chemical element with unstable neutron number, so that it undergoes spontaneous nuclear disintegration. Major use in biology is to trace the fate of atoms or molecules that follow the same metabolic pathway or enzymic fate as the normal stable isotope, but that can be detected with high sensitivity by their emission of radiation. Also used to locate the position of the radioactive metabolite, as in autoradiography and to measure relative rates of synthesis of compounds from radioactive precursors. Radiolabeling: Attachment of radionuclides to biomolecules such as proteins. Radionuclides: A radioactive nuclide that disintegrates with the emission of electromagnetic radiation. Radiosensitizer: A chemotherapeutic agent used to enhance the effect of radiation therapy. Radiotherapists: Physicians specially trained to use radiation to treat patients. R as proteins: Small GTP-binding proteins encoded by ras genes that play a critical role in normal cellular growth, differentiation, and development, and have the potential for malignant transformation. Two of the major ras proteins include the normal cellular form, proto-oncogene protein p21(ras), and the malignant form, oncogene protein p21(ras). Receptor: A molecular structure within a cell or on the surface characterized by selective binding of a specific substance and a specific physiologic effect that accompanies the binding. Receptor antagonist: Agent that tends to nullify the action of another, as a drug that binds to the cell receptor without eliciting a biological response. Recessive: The gene that exerts its effect when presents in both homologous chromosomes. If present in one chromosome is masked by the dominant effect of the gene present in the other chromosome. As shown below many diseases are associted with recessive genes. Recombinant virus: A virus formed by the introduction of foreign gene into its genome by genetic engineering technique. Recombination: In genetics, the formation of new combinations of genes as a result of crossing over between chromosomes. This happens routinely during the process of meiosis. Redox status: Oxidation-reduction status. Redox system: Oxidation-reduction system, an enzyme system in the tissues by which oxidation and reduction proceed simultaneously through the transference of hydrogen or of one or more electrons from one metabolite to another. Reflux: A backward or return flow, for example reflux of the stomach or duodenal content. Refractory: Not readily yielding to treatment. Reticulocytes: Young red blood cells showing network of membranous structures when stained with basic dyes. Retinoblastoma: Malignant tumor of the retina, usually arising in the inner nuclear layer of the neural retina. Retinoblastoma is unusual in being caused by an autosomal dominant mutation in some cases (about 6%), in which case it may be bilateral. The gene product of the retinoblastoma gene is a tumor suppressor that interacts with transcription factors such as E2F to block transcription of growth regulating genes. The Rb gene plays a role in normal development, not just that of the retina. Retinoids: Derivatives of retinal, aldehyde of retinol. Retinol is obtained by oxidative enzymatic splitting of absorbed dietary carotene, and having vitamin A activity. Retinopathy: Inflammatory or noninflammatory conditions of the retina, the innermost layer of the eyeball. Retro-: A prefix backward, or locate behind. Retrospective data: When data are collected from past experience. Retroviral vector: Attenuated retrovirus that is used to introduce DNA material (gene) into cell. Retrovirus: A large group RNA viruses that are so called because they carry reverse transcriptase. This enzyme can copy RNA to form a DNA against the "Dogma". Many cancers in vertebrates are caused by retroviruses. Rheumatoid arthritis: A chronic inflammatory lesion primarily of the joints. Rhodopsin: Light sensitive pigment formed from retinal linked to opsin; rhodopsin is an integral membrane protein found in the discs of retinal rods and cones, comprising some 40% of the membrane. Vertebrate opsins are proteins of 38 kD. Ribosome: A large molecular structure having two dissociable subunits that is the site of protein synthesis. Several ribosomal RNA (rRNA) and many proteins are part of this unit. The ribosomes come with one large subunit and a smaller one. Each contains RNAs and proteins. RNA: Ribonucleic acid, a nucleic acid molecule similar to DNA but contains ribose rather than deoxyribose. RNA is formed on DNA template. There are several classes of RNA: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). They play crucial role in protein synthesis and other cell activities. RNA virus: Viruses with RNA as genome. RNases: Ribonucleases, widely distributed type of enzymes that cleave RNAs. May act as endonucleases or exonucleases depending upon the type of enzyme. Generally recognise target by tertiary structure rather than sequence. RNase A: One group of hydrolytic enzymes that catalyzes the hydrolysis of phosphate ester linkages in ribonucleic acids.