somatic hypermutation In immunology: accumulation of point mutations in the assembled variable-region-coding sequences of immunoglobulin genes that occurs when B cells are activated to form memory cells. Results in the production of antibodies with altered antigen-binding sites, some of which bind antigen with increased affinity; it is responsible for affinity maturation in antibody responses.
somatic mutations   In cancer, one or more detectable abnormalities in the DNA sequence of tumor cells that distinguish them from the normal somatic cells surrounding the tumor.
somite   One of a series of paired blocks of mesoderm that form during early development and lie on either side of the notochord in a vertebrate embryo. They give rise to the segments of the body axis, including the vertebrae, muscles, and associated connective tissue. (Figure 21–38) sorting signal   Signal sequence or signal patch that directs the delivery of a protein to a specific location, such as a particular intracellular compartment.
spectrin   Abundant protein associated with the cytosolic side of the plasma membrane in red blood cells, forming a network that supports the membrane. Also present in other cells.  (Figure 10–38)
S phase Period of a eukaryotic cell cycle in which DNA is synthesized. (Figure 17–4) spinal cord   Bundle of neurons and support cells that extends from the brain.
spindle assembly checkpoint Regulatory system that operates during mitosis to ensure that all chromosomes are properly attached to the spindle before sister-chromatid separation starts. (Figure 17–9 and Panel 17–1, pp. 980–981) spliceosome   Large assembly of RNA and protein molecules that performs pre-mRNA splicing in eukaryotic cells.
Src (Src protein family) Family of cytoplasmic tyrosine kinases (pronounced “sark”) that associate with the cytoplasmic domains of some enzyme-linked cell-surface receptors (for example, the T cell antigen receptor) that lack intrinsic tyrosine kinase activity. They transmit a signal onward by phosphorylating the receptor itself and specific intracellular signaling proteins on tyrosines. (Figure 3–10)
SRP (signal-recognition particle) receptor Component in the endoplasmic reticulum (ER) membrane that guides the signal recognition particle to the ER membrane.
starch Polysaccharide composed exclusively of glucose units, used as an energy-storage material in plant cells. (Figure 2–51)
Start (restriction point) Important transition at the end of G1 in the eukaryotic cell cycle. Passage through Start commits the cell to enter S phase. The term was originally used for this point in the yeast cell cycle only; the equivalent point in the mammalian cell cycle was called the restriction point. In this book we use Start for both. (Figure 17–9) start-transfer signal Short amino acid sequence that enables a polypeptide chain to start being translocated across the endoplasmic reticulum membrane through a protein translocator. Multipass membrane proteins sometimes have both N-terminal (signal sequence) and internal start-transfer signals. (Figure 12–42)
STAT (signal transducer and activator of transcription) Latent transcription regulator that is activated by phosphorylation by Janus kinases (JAKs) and enters the nucleus in response to signaling from receptors of the cytokine receptor family. (Figure 15–56) stem cell   Undifferentiated cell that can continue dividing indefinitely, throwing off daughter cells that can either commit to differentiation or remain a stem cell (in the process of self-renewal). (Figure 22–3) stem-cell niche   The specialized microenvironment in a tissue in which self-renewing stem cells can be maintained.
steroid hormones   Hormones, including cortisol, estrogen, and testosterone, that are hydrophobic lipid molecules derived from cholesterol that activate intracellular nuclear receptors.