Amino acids -- A group of small molecules that join to form proteins, amino acids are often referred to as the building blocks of proteins. Although more than 100 amino acids occur naturally, only 20 are usually involved in the synthesis of protein -- and humans can synthesize at least 100,000 different proteins. The 20 amino acids have the same backbone, but each has a separate side chain -- known as an R-group -- that distinguishes it from the others.

Antibody -- A protein produced by the immune system that helps the body fight a particular disease or develop an immunity to it. The human body is capable of generating more than a trillion different antibodies.

Base pair -- The basic units of DNA and RNA, base pairs are chemical structures made up of the chemicals adenine, thymine, guanine and cytosine, which are designated by the letters A, T, G and C, respectively. Adenine always pairs with thymine and guanine always pairs with cytosine, creating the pairs or nucleotides in which genetic information is found.

Cell -- The basic unit of any living organism. It is a small, watery compartment filled with chemicals and a complete copy of the organism's genome.

Chromosome -- One of the threadlike "packages" of genes and other DNA that is compressed and wrapped around protein in the nucleus of a cell. Different kinds of organisms have different numbers of chromosomes. Humans have 23 pairs of chromosomes, 46 in all.

DNA -- Deoxyribonucleic acid. The material inside the nucleus of a cell that carries the genetic instructions for making living organisms.

Double helix -- The spiraling lattice of double-stranded DNA that contains the genetic information pertinent to the organism. The sides of the lattice -- its spine -- are composed of sugar and phosphate molecules. The crosspieces, or rungs, are made up of base pairs, also known as nucleotides.

Gene -- The basic unit of heredity, the gene contains the functional and physical characteristics passed from parent to offspring. Genes are pieces of DNA, and most genes contain the information for making a specific protein.

Gene mapping -- The process of determining the positions of genes on a chromosome and the distance between them.

Gene therapy -- The introduction of healthy genetic material to replace, augment or influence genes that do not function properly. In some cases the material can be injected with what is known as a genetic vaccination. In other cases the material is introduced through bioengineered viruses that carry the therapeutic gene to the cell. Globules known as liposomes can also be used to carry therapeutic genes to specific cells.

Genetic code -- The instructions in a gene that tell the cell how to make a specific protein. A, T, G and C are the "letters" of the DNA code; they stand for the chemicals adenine, thymine, guanine and cytosine, respectively, which are the basic chemical units of DNA. Each gene combines the four chemicals in various ways to spell out 3-letter "words" that specify which amino acid is needed at every step in making a protein. The precise order that spells out these "words" is the genetic code.

Genome -- All the DNA in an organism, including its genes. The DNA is found as tightly coiled threads in the nucleus of every cell. The threads are composed of paired strands of nucleotides or base pairs. There are 3.2 billion base pairs in the human genome and 80,000 to 100,000 genes.

Mutation -- A permanent structural alteration in DNA. In most cases such DNA changes either have no effect or cause harm. Occasionally, however, a mutation can improve an organism's chance of surviving and passing the beneficial change on to its descendants.

Nucleus -- The central cell structure that houses the chromosomes.

Protein -- A large complex molecule made up of one or more chains of amino acids. Proteins perform a wide variety of activities in the cell.

RNA -- Ribonucleic acid. Sometimes confused with DNA, RNA is a group of nucleic acids that along with DNA comprises the genetic material of the cell. Genetic information is stored by DNA in the nucleus of cells, and RNA carries that information to other parts of the cell where it is converted into protein.

Sequencing -- The process of identifying the order in which chains of repeating units of base pairs appear in DNA and identifying the order of amino acids in proteins. Researchers label copies of a DNA sequence with fluorescent markers, then run them through a sequencing machine. In proteins, amino acids are removed one at a time from the end of a protein and identified with an automated system.