homologous chromosomes (homologs) The maternal and paternal copies of a particular chromosome in a diploid cell.
homologous recombination (general recombination) Genetic exchange between a pair of identical or very similar DNA sequences, typically those located on two copies of the same chromosome. Also a DNA repair mechanism for double-strand breaks. (Figures 5–48, 5–50, and 5–54) homophilic Binding between molecules of the same kind, especially those involved in cell–cell adhesion. (Figure 19–5) horizontal gene transfer Gene transfer between bacteria via natural transformation by released naked DNA, transduction by bacteriophages, or sexual exchange by conjugation.
hormone   Signal molecule secreted by an endocrine cell into the bloodstream, which can then carry the signal to distant target cells.
Hox complex A gene complex consisting of a series of Hox genes
Hox genes   Genes coding for transcription regulators, each gene containing a homeodomain, and specifying body-region differences. Hox mutations typically cause homeotic transformations.
Hox proteins   Transcription regulator proteins encoded by Hox genes; possess a highly conserved, 60-amino-acid-long DNA-binding homeodomain.
HPV Human papillomavirus; infects the cervical epithelium and is important as a cause of carcinoma of the uterine cervix.
hyaluronan (hyaluronic acid) Type of nonsulfated glycosaminoglycan with a regular repeating sequence of up to 25,000 identical disaccharide units, not linked to a core protein. Found in the fluid lubricating joints and in many other tissues. (Figures 19–33 and 19–34) hybridization   In molecular biology, the process whereby two complementary nucleic acid strands form a base-paired duplex DNA-DNA, DNA-RNA, or RNA-RNA molecule. Forms the basis of a powerful technique for detecting specific nucleotide sequences. (Figures 5–47 and 8–33) hybridoma Hybrid cell line generated by fusion of a tumor cell and another cell type. Monoclonal antibodies are produced by hybridoma lines obtained by fusing antibody-secreting B cells with cells of a B lymphocyte tumor. (Figure 8–4) hydrogen bond Noncovalent bond in which an electropositive hydrogen atom is partially shared by two electronegative atoms. (Panel 2–3, pp. 94–95) hydronium ion (H3O+) Water molecule associated with an additional proton. The form generally taken by protons in aqueous solution.
hydrophilic   Dissolving readily in water. Literally, “water loving.” hydrophobic (lipophilic) Not dissolving readily in water. Literally, “water-fearing.” hydrophobic force   Force exerted by the hydrogen-bonded network of water molecules that brings two nonpolar surfaces together by excluding water between them. (Panel 2–3, pp. 94–95) hypervariable region   In immunology: any of the three small parts of the variable region of an immunoglobulin or T cell receptor chain that show the highest variability from molecule  to molecule and contribute to the antigen-binding site. (Figure 24–26) hypoxia-inducible factor 1α (HIF1α). Transcription regulator, the intracellular levels of which increase in response to a shortage of oxygen, that stimulates transcription of the VEGF gene to promote angiogenesis.
IκB   Inhibitory proteins that bind tightly to NFκB dimers and hold them in an inactive state within the cytoplasm of unstimulated cells.
Ig superfamily   Large and diverse family of proteins that contain immunoglobulin or immunoglobulin-like domains. Most are involved in cell–cell interactions or antigen recognition. (Figure 24–48)