Toll-like receptors (TLRs) Family of pattern recognition receptors (PRRs) on or in cells of the innate immune system. They recognize pathogen-associated immunostimulants (PAMPs) associated with microbes. (Figure 24–4)
TOM complex   Multisubunit protein complex that transports proteins across the mitochondrial outer membrane.  (Figure 12–21)
TOR   Large, serine/threonine protein kinase that is activated by the PI-3-kinase–Akt signaling pathway and promotes cell growth.
totipotent Describes a cell that is able to give rise to all the different cell types in an organism.
trans face Face on the other (far) side.
trans Golgi network (TGN) Network of interconnected tubular and cisternal structures closely associated with the trans face of the Golgi apparatus and the compartment from which proteins and lipids exit the Golgi, bound for the cell surface or another compartment.
transcellular transport   Transport of solutes, such as nutrients, across an epithelium, by means of membrane transport proteins in the apical and basal faces of the epithelial cells. (Figure 11–11) transcription (DNA transcription) Copying of one strand of DNA into a complementary RNA sequence by the enzyme RNA polymerase. (Figures 6–1 and 6–8) transcription regulators   General name for any protein that binds to a specific DNA sequence (known as a cis-regulatory sequence) to influence the transcription of a gene.
transcriptional control Regulation by a cell of gene expression by controlling when and how often a given gene is transcribed.
transcytosis Uptake of material at one face of a cell by endocytosis, its transfer across a cell in vesicles, and discharge from another face by exocytosis. (Figure 13–58) transfer RNA (tRNA) Set of small RNA molecules used in protein synthesis as an interface (adaptor) between mRNA and amino acids. Each type of tRNA molecule is covalently linked to a particular amino acid. (Figure 6–50) transferrin receptor   Cell-surface receptor for transferrin (a soluble protein that carries iron) that delivers iron to the cell interior via receptor-mediated endocytosis and recycling of the receptor–transferrin complex.
transformed   A cell with an altered phenotype that behaves in many ways like a cancer cell (i.e., unregulated proliferation, anchorage-independent growth in culture).
transforming growth factor-β superfamily (TGFβ superfamily) Large family of structurally related secreted proteins that act as hormones and local mediators to control a wide range of functions in animals, including during development. It includes the TGFβ/activin and bone morphogenetic protein (BMP) subfamilies. (Figure 15–57) transgene   The foreign or modified gene that has been added to create a transgenic organism.
transgenic organism Plant or animal that has stably incorporated one or more genes from another cell or organism (through insertion, deletion, and/or replacement) and can pass them on to successive generations. (Figures 8–53 and 8–70) transit amplifying cell   Cell derived from a stem cell that divides a limited number of times before terminally differentiating.
transition state   Structure that forms transiently in the course of a chemical reaction and has the highest free energy of any reaction intermediate. Its formation is a rate-limiting step in the reaction. (Figure 3–47) translation (RNA translation) Process by which the sequence of nucleotides in an mRNA molecule directs the incorporation of amino acids into protein. Occurs on a ribosome. (Figures 6–1 and 6–64) translational control   Regulation by a cell of gene expression by selecting which mRNAs in the cytoplasm are translated by ribosomes.
translocon The assembly of a translocator associated with other membrane complexes, such as enzymes that modify the growing polypeptide chain.