Terms Starting with Letter: H half-life Radioactive decay is characterized by the disappearance of a constant fraction of the activity present in the sample during a given time interval. The half-life of a radionuclide is the time required for it to decay to half of its initial activity level. haploid An organism or cell having only a single member of each pair of homologous chromosomes resulting in half the full set of genetic material produced in meiosis. A haploid set of chromosomes is present in the reproductive cells of plants and animals. For humans, with 46 chromosomes, a haploid set would be 23. herpes simplex virus (HSV) A DNA virus of the family Herpesviridae. HSV thymidine kinase is used in molecular imaging as a reporter gene as well as a suicide gene in gene therapy. heterozygous The presence of different alleles of a given gene on the pair of homologues carrying that gene. high-thoughput screening Pertaining to various new technologies that allow one to study large numbers of interactions between chemicals and their intended targets and efficiently categorize them. homologous chromosomes A pair of chromosomes in a diploid cell having the same genetic composition. Each is derived from a different parent. hot nodule A nodule with a higher uptake of radioactive tracer than the surrounding cells. human serum albumin (HSA) Most abundant blood protein. humanization The reduction of immunogenicity of a nonhuman protein sequence without altering its intended functionality by adapting it to the amino acid sequence of a human protein. hybridization The association of two complementary nucleic acid strands (either two DNA, two RNA or one DNA and one RNA) to form a double-stranded, helical molecule. hypertrophy increase in size hypoechoic An area in an ultrasound scan where the echoes are weaker or less than normal.