GTPase   An enzyme that converts GTP to GDP. GTPases fall into two large families. Large trimeric G proteins are composed of three different subunits and mainly couple GPCRs to enzymes or ion channels in the plasma membrane. Small monomeric GTP-binding proteins (also called monomeric GTPases) consist of a single subunit and help relay signals from many types of cell-surface receptors and have roles in intracellular signaling pathways, regulating intracellular vesicle trafficking, and signaling to the cytoskeleton. Both trimeric  G proteins and monomeric GTPases cycle between an active GTP-bound form and an inactive GDP-bound form and frequently act as molecular switches in intracellular signaling pathways. See page 820.
GTPase-activating protein (GAP) Protein that binds to a GTPase and inhibits it by stimulating its GTPase activity, causing the enzyme to hydrolyze its bound GTP to GDP.  (Figure 15–8) guanine nucleotide exchange factor (GEF) Protein that binds to a GTPase and activates it by stimulating it to release its tightly bound GDP, thereby allowing it to bind GTP in its place. (Figure 15–8) haplotype block Combination of alleles and DNA markers that has been inherited in a large, linked block on one chromosome of a homologous pair—undisturbed by genetic recombination—across many generations.
Hedgehog protein   Secreted extracellular signal molecule that has many different roles controlling cell differentiation and gene expression in animal embryos and adult tissues. Excessive Hedgehog signaling can lead to cancer.
helper T cell (TH cell) Type of T cell that helps activate B cells to make antibodies, cytotoxic T cells to become effector cells, and macrophages to kill ingested pathogens. They can also help activate dendritic cells.
heterochromatin   Chromatin that is highly condensed even in interphase; generally transcriptionally inactive. (Compare with euchromatin.) heterochronic Describes genes involved in developmental timing; mutation results in cells of a specific fate behaving as cells at a different stage of development.
high-mannose oligosaccharides   Broad class of N-linked oligosaccharides, attached to mammalian glycoproteins in the endoplasmic reticulum, containing two N-acetylglucosamine residues and many mannose residues.
high-performance liquid chromatography (HPLC) Type of chromatography that uses columns packed with tiny beads of matrix; the solution to be separated is pushed through under high pressure.
histone   One of a group of small abundant proteins, rich in arginine and lysine, that combine to form the nucleosome cores around which DNA is wrapped in eukaryotic chromosomes. (Figure 4–24) histone chaperone (chromatin assembly factor) Protein that binds free histones, releasing them once they have been incorporated into newly replicated chromatin. (Figure 4–27) histone H1    “Linker” (as opposed to “core”) histone protein that binds to DNA where it exits from a nucleosome and helps package nucleosomes into the 30-nm chromatin fiber.  (Figure 4–30)
Holliday junction (cross-strand exchange) X-shaped structure observed in DNA undergoing recombination, in which the two DNA molecules are held together at the site of crossing-over, also called a cross-strand exchange.  (Figure 5–55) homeotic selector gene In Drosophila development, a gene that defines and preserves the differences between body segments.
homolog   One of two or more genes that are similar in sequence as a result of derivation from the same ancestral gene. The term covers both orthologs and paralogs. (Figure 1–21) See homologous chromosomes.
homologous   Genes, proteins, or body structures that are similar as a result of a shared evolutionary origin.