AIDS, or acquired immune deficiency syndrome -- A disease of the human immune system that is categorized by a reduction in the number of helper T cells to 20 percent or less of normal, rendering the person highly vulnerable to life-threatening conditions and to some that become life-threatening, and that is caused by infection by the HIV virus.

AMP, or Adenosine monophosphate -- One of the four nucleotides in an RNA molecule. Two phosphates are added to AMP to form ATP.

DNA, or deoxyribonucleic acid -- The primary genetic material of all cellular organisms and the DNA viruses. Located predominantly in cell nuclei, it is composed of two chains of nucleotides--deoxyribose and phosphate backbones with side chains of purine (adenine or guanine) or pyrimidine (cytosine and thymine) bases projecting inward. Hydrogen bonds link adenine to guanine, and cytosine to thymine. The two linked strands are twisted in a double helix.

DNA-RNA hybridization -- A laboratory technique used to form a double strand of DNA out of a single strand of DNA and a complementary strand of RNA. The double strand of the original DNA is "unzipped" into single strands and then exposed to a complementary strand of RNA. The RNA then bonds with a strand of the DNA. Such hybridization is highly specific and, therefore, enables identification of different types and sequences of DNA and RNA. It is essential to analyzing the genome, and in the effective use of recombinant DNA technology. Also called molecular hybridization.

Embryo -- An animal in the early stages of growth and differentiation that are characterized by cleavage, the laying down of fundamental tissues, and the formation of primitive organs and organ systems. In humans, the embryo is this developing organism from approximately the fourth day after fertilization to the end of the eighth week of gestation.

Escherichia coli, or E. coli -- A common bacterium that has been studied intensively by geneticists because of its small genome size, normal lack of pathogenicity, and ease of culture in the laboratory.

Hepatitis B virus -- A sometimes fatal virus caused by a double-stranded DNA virus that tends to persist in the blood serum and is transmitted especially by contact with infected blood or by contact with other infected bodily fluids.

HIV, or human immunodeficiency virus -- Any of several retroviruses and especially HIV-1 that infect and destroy helper T cells of the immune system causing the marked reduction in their numbers that leads to AIDS.

Homology -- Similarity of nucleotide or amino acid sequences in nucleic acids, peptides, or proteins. Also: structural similarity resulting from descent from a common form.

Mutation -- The process that produces a gene or a chromosome that differs from the wild-type. This process, either through an alteration in the nucleotide sequence of the DNA coding for a gene or through a change in the physical arrangement of a chromosome, results in the creation of a new character or trait not found in the parental type.

Oncogenes -- Genes having the potential to cause a normal cell to become cancerous.

Retroviruses, or retroviridae -- A family of single-stranded RNA viruses that produce the enzyme reverse transcriptase by means of which DNA is synthesized using their RNA as a template, and incorporated into the genome of infected cells. After incorporation, the virus may not be expressed by remain in a latent state (proviruses).

RNA, or ribonucleic acid -- A single-stranded nucleic acid found in the cell nucleus and cytoplasm, which plays a key role in protein synthesis. (It also constitutes the genetic material of the RNA viruses.) It is similar to DNA but has ribose sugar, rather than deoxyribose sugar, and uracil, rather than thymine, as one of the pyrimidine bases. There are several classes of RNA molecules, including messenger RNA, transfer RNA, and ribosomal RNA, each serving a different purpose in the cell.

RNA, viral -- Ribonucleic acid that makes up the genetic material of viruses.

Rous sarcoma virus -- A retrovirus that causes malignant sarcoma tumors in chickens.

Stem cell -- An unspecialized cell in developing embryos that gives rise to differentiated cells.

Virus -- Any one of a group of minute infectious agents that are not visible under a light microscope and are characterized by a lack of independent metabolism and an ability to replicate only within living host cells. Even though they are not living organisms, like living organisms they have the ability to pass genes through multiple generations and to mutate. The individual virus particle consists of a strand or strands of nucleic acid (which may be either DNA or RNA) and a protein shell. Viruses are classified into three main subgroups based on their host, bacterial viruses, plant viruses, and animal viruses. Viruses are then further classified by their origin, means of transmission, or illnesses they produce.