Acquired Mutations: Gene changes that arise within individual cells, due to various causes, and accumulate throughout a person’s lifetime. Acquired mutations are also called somatic mutations. (See Germline Mutation).

Adenocarcinoma: A type of cancer involving the cells from the lining or inner surface of an organ such as the colon. Most cancers of the colon, breast, and lung are adenocarcinomas.

Adenoma: A benign tumor involving cells from the lining or inner surface of an organ, such as the colon. If an adenoma becomes cancerous, it is then called an adenocarcinoma.

Agonist: A chemical that mimics the biochemical action of another chemical. Agonists often closely resemble naturally occurring substances and cause the same response in a cell that the naturally occurring substance would. The opposite is an antagonist, which blocks the action of an agonist.

Allele: A variant form or version of the same gene. Different alleles produce variations in inherited characteristics, which can be benign or disease-causing. An example of benign allele variations are those that cause different eye colors or blood types.

Alpha Fetoprotein: Alpha fetoprotein (AFP) is a protein normally produced by the liver and yolk sac of a fetus. AFP levels decrease soon after birth. It probably has no normal function in adults. [Source]

Amino Acid: Any of a class of 20 molecules that combine to form proteins.

Amniocentesis: A prenatal diagnosis method which uses cells from the amniotic fluid surrounding a developing fetus to determine the number and structure of its chromosomes, biochemical testing, or mutation detection. Amniocentesis is usually performed at 15 to 18 weeks into the pregnancy, calculated from the Last Menstrual Period. The procedure involves inserting a fine needle through the mother’s abdomen and uterus into the amniotic fluid surrounding the baby while using an ultrasound scan for guidance to prevent injury to the fetus. One to 2 tablespoons (20-30 ml) of the amniotic fluid are removed for analysis.

Amniocentesis is generally recommended when there is a family history of a genetic disorder or when the woman is over age 35 and therefore at a higher risk of having a baby with a chromosomal abnormality. It is a highly accurate test for chromosomal abnormalities, such as Trisomy 21 (Down Syndrome). The procedure is not very painful and has been described as a ‘cramping’ feeling. When performed at a qualified center, the risk of complications is ~0.5% and includes bleeding, cramping, and pregnancy loss. For other types of prenatal diagnosis, see Chorionic Villus Sampling, Ultrasound, Percutaneous Umbilical Blood Sampling,and Preimplantation Genetic Diagnosis.

Amyloid: A starchlike protein (known as a glycoprotein) that is deposited in tissues as a part of a disease process, called amyloidosis.

Amyloidosis: Disease resulting from the deposition of amyloid, a starchlike protein (known as a glycoprotein), in various tissues or organs of the body and impairing their function.

Aneuploidy: One or a few chromosomes above or below the normal chromosome number. In human beings, 23 pairs of chromosomes is the normal chromosome number in each non-reproductive cell. Disease can occur when there are more or less chromosomes than normal, or anueploidy. For example, three copies of chromosome number 21 or trisomy 21 (Down syndrome) is a form of aneuploidy.

Angiogenesis: blood vessel formation or growth. For cancer, it describes the formation of blood vessels between a tumor and the surrounding tissues. This process is necessary in order to keep the tumor alive and to allow it to grow larger.

Antagonist: A substance that acts against and blocks the biochemical action of a chemical. For example, a drug which binds to a cell receptor without eliciting the normal biochemical response is an antagonist. It is the opposite of an agonist.

Apoptosis: A form of cell death in which the cell uses specialized cellular machinery to kill itself. This is a normal cellular process and is necessary for development and health of the body by eliminating old cells, unnecessary cells, and unhealthy cells. Without an orderly process for getting rid of cells, we would double in size within a year. Cancerous cells do not go through apoptosis although they are unhealthy. Apoptosis is also called programmed cell death.

Aromatase: An enzyme involved in the production of estrogen. For women who have gone through menopause, aromatase produces all of the estrogen in the body.

Ashkenazi Jew: A person who has Jewish ancestors from Central and Eastern Europe including Germany, Austria, Poland, Lithuania, Russia, etc. Eighty percent of American Jews are of Ashkenazi descent.

Ataxia: A neurologic term describing muscular incoordination often manifesting as an unsteady gait and lack of postural balance.

Atypical Hyperplasia: An increase in the number of normal cells in a tissue. This has been seen in breast tissue and often leads to a mass, which a physician would biopsy to see if it is benign or malignant. See Factors Contributing to Breast Cancer Risk.

Autosomal Dominant Inheritance: A pattern of inheritance where an alteration in just one copy of a pair of genes causes a detectable trait. Individuals who possess one mutant copy of a gene for a dominant disorder are usually affected. A child conceived by an affected individual has a 50% chance of inheriting the abnormal copy of the gene, and thus the disorder or trait. Dominant disorders do not “skip” generations and most affected individuals have an affected parent.

There are two exceptions to this: 1) a new mutation in a dominant gene can cause the trait to appear for the first time in a family; 2) a few genes may have incomplete penetrance, i.e. not everyone who has the gene manifests the trait. Torsion dystonia is a dominantly inherited condition with reduced penetrance. For more information, see Introduction to Genetics – Autosomal Dominant Inheritance.

Autosomal Recessive Inheritance: A pattern of inheritance where a specific trait is manifest only when both copies of the gene are altered. A disorder is considered recessive when a single fully functional copy of a gene is sufficient to prevent disease.

In order for an individual to have two non-functional copies of a gene, a mutated copy must be inherited from each parent. It is expected that both parents of an affected child are entirely unaffected, even though they both carry an altered copy of the gene; they are called “carriers.” When both partners in a couple are carriers, they have a 25% (1-in-4) chance of having an affected child with each pregnancy. For more information, see Introduction to Genetics – Autosomal Recessive Inheritance. An example of an autosomal recessive disease is Tay-Sachs disease.

Autosome: Any of the non-sex-determining chromosomes. Human cells have 22 pairs of autosomes, numbered 1 to 22. (In addition to autosomes, human cells also contain a pair of sex chromosomes – XX in females and XY in males).

Axillary Lymph Node: Lymph nodes that are located in the cavity beneath the junction of the arm and the body (armpit). These lymph nodes are often biopsied in women with breast cancer to look for metastases.

Azoospermia: A condition in which there is a lack of sperm in the semen. Men with this condition often have cystic fibrosis transmembrane conductance regulator (CFTR) gene mutations. CFTR mutations can cause Cystic Fibrosis, in which the vast majority of men are infertile.