regulatory T cell (Treg) A type of T cell that suppresses the development, activation, or function of other immune cells via secreted cytokines or cell-surface inhibitory proteins.
replication fork   Y-shaped region of a replicating DNA molecule at which the two strands of the DNA are being separated and the daughter strands are being formed. (Figures 5–7 and 5–18) replication origin Location on a DNA molecule at which duplication of the DNA begins. (Figures 4–19 and 5–23) replicative cell senescence Phenomenon observed in primary cell cultures in which cell proliferation slows down and finally irreversibly halts.
respiratory chain (electron-transport chain) Electron-transport chain present in the inner mitochondrial membrane that generates an electrochemical gradient across the membrane that is used to drive ATP synthesis.  (Figures 14–4 and 14–10) resting membrane potential   Electrical potential across the plasma membrane of a cell at rest, i.e. a cell that has not been stimulated to open additional ion channels than those that are normally open.
restriction nuclease One of a large number of nucleases that can cleave a DNA molecule at any site where a specific short sequence of nucleotides occurs. Extensively used in recombinant DNA technology. (Figure 8–24) restriction point Important transition at the end of G1 in the eukaryotic cell cycle; commits the cell to enter S phase. The term was originally used for this transition in the mammalian cell cycle; in this book we use the term Start. (Figure 17–9) retinoblastoma   A rare type of human cancer arising from cells in the retina of the eye that are converted to a cancerous state by an unusually small number of mutations. Studies of retinoblastoma led to the discovery of the first tumor suppressor gene.
retinoblastoma protein (Rb protein) Tumor suppressor protein involved in the regulation of cell division. Mutated in the cancer retinoblastoma, as well as in many other tumors. Its normal activity is to regulate the eukaryotic cell cycle by binding to and inhibiting the E2F proteins, thus blocking progression to DNA replication and cell division. (Figure 17–61) retroviral-like retrotransposons A large family of transposons that move themselves in and out of chromosomes by a mechanism similar to that used by retroviruses, being first transcribed into an RNA copy that is converted to DNA by reverse transcriptase then inserted elsewhere in the genome. (Table 5–4, p. 288) retrovirus   RNA-containing virus that replicates in a cell by first making an RNA–DNA intermediate and then a double-strand DNA molecule that becomes integrated into the cell’s DNA. (Figure 5–62) reverse genetics   Approach to discovering gene function that starts from the DNA (gene) and its protein product and then creates mutants to analyze the gene’s function.
reverse transcriptase   Enzyme first discovered in retroviruses that makes a double-strand DNA copy from a single-strand RNA template molecule.
RGD sequence   Tripeptide sequence of arginine-glycineaspartic acid that forms a binding site for integrins; present in fibronectin and some other extracellular proteins. (Figure 19–47C)
Rheb   A monomeric Ras-related GTPase that in its active form (Rheb-GTP) activates mTOR, which promotes cell growth.
Rho Member of the Rho family of monomeric GTPases that regulate the actin and microtubule cytoskeletons, cell-cycle progression, gene transcription, and membrane transport.