H

Haploid: A single set of chromosomes (half the full set of genetic material), present in the egg and sperm cells of animals and in the egg and pollen cells of plants. Human beings have 23 chromosomes in their reproductive cells. Compare diploid.

Halsted radical mastectomy: Removal of the breast, skin, nipple, areola, both pectoral muscles, and all axillary lymph nodes on the same side.

Hematologist: Doctor specializing in diseases of the blood and blood-forming tissues.

Hematoma: Collection of blood outside a blood vessel caused by a leak or injury. 

HER2/neu gene: This "oncoprotein" (cancer protein) is present in very small amounts on the surface of normal breast cells. HER stimulates cell growth, and breast cancers that produce too much of this protein tend to be more aggressive. About 30% of breast cancers have too much of this protein. A monoclonal antibody that attaches to the HER2 protein slows the growth of breast cancer cells and may stimulate the immune system to more effectively attack cancer. Some other types of cancer also have too much HER2/neu protein. Studies of monoclonal antibody therapy for these cancers are in progress. 

Hereditary nonpolyposis colon cancer (HNPCC): Hereditary tendency to develop cancer at a young age without first having many polyps. 

Heterozygosity: The presence of different alleles at one or more loci on homologous chromosomes.

High risk: Greater than general population's normal chance of developing cancer. High risk factors may include: heredity (family history of breast, colon, prostate or other cancers), personal habits (smoking, heavy drinking, etc.), or environmental (overexposure to sunlight or toxic materials).

Hodgkin's disease: Often curable cancer affecting lymphatic system. (Named after the doctor who first identified it.)

Home health nurse: Nurse who gives medications in the home, teaches patients how to care for themselves, and assesses their condition to see if further medical attention is needed.

Homeobox: A short stretch of nucleotides whose base sequence is virtually identical in all the genes that contain it. It has been found in many organisms from fruit flies to human beings. In the fruit fly, a homeobox appears to determine when particular groups of genes are expressed during development.

Homology: Similarity in DNA or protein sequences between individuals of the same species or among different species.

Homologous: Similar in structure and origin but not necessarily function.  For example the wing of a bird, forelimb of a monkey, and arm of a man are said to be "homologous". 

Homologous chromosome: Chromosome containing the same linear gene sequences as another, each derived from one parent.

Hormone: Chemical released by endocrine glands (thyroid, adrenal, ovaries, etc.). Hormones travel through bloodstream and set in motion various functions.

Hormone receptor: Protein on a cell surface (or within the cell cytoplasm) that binds a hormone. Tumors can be tested for hormone receptors to see if they can be treated with hormones or anti-hormones. (See: Hormone receptor assay.)

Hormone receptor assay: Test to see if a breast tumor is likely to be affected by or treated with hormones. (See: Estrogen receptor assay, Progesterone receptor assay.)

Hormone replacement therapy: Use of estrogen and progesterone from an outside source after the body has stopped making them. This therapy is often given to relieve symptoms of menopause and has been shown to offer protection against heart disease and thinning of bones (osteoporosis) in women after menopause. Estrogen, however, also nourishes some types of breast cancer. Scientists, therefore, are continuing to research just how serious the increased breast cancer risk is compared to the risks of heart disease and osteoporosis.

Hormone therapy: Treatment with hormones, with drugs that interfere with hormone production or hormone action, or the surgical removal of hormone-producing glands. Hormone therapy may kill cancer cells or slow their growth.

Hospice: Special care for people in the final phase of illness as well as care for their families and caregivers. The care may take place in the patient's home or in a homelike facility.

Human gene therapy: Insertion of normal DNA directly into cells to correct a genetic defect.

Human Genome Initiative: Collective name for several projects begun in 1986 by DOE to (1) create an ordered set of DNA segments from known chromosomal locations, (2) develop new computational methods for analyzing genetic map and DNA sequence data, and (3) develop new techniques and instruments for detecting and analyzing DNA. This DOE initiative is now known as the Human Genome Program. The national effort, led by DOE and NIH, is known as the Human Genome Project.

Hybridization: The process of joining two complementary strands of DNA or one each of DNA and RNA to form a double-stranded molecule.

Hyperalimentation: Provision of nutrition other than as food, often intravenously. 

Hyperplasia: Too much growth of cells or tissue in a specific area, such as the lining of the prostate. (See: Benign prostatic hyperplasia.)

Hyperthermia therapy: Raising body temperature to treat a disease.

Hysterectomy: Surgical removal of the uterus through an incision in the abdomen or through the vagina. Removal of the ovaries (oophorectomy) may be done at the same time.