The precise arrangement (sequence) of A, C, G, and T bases on a DNA strand is the recipe that encodes the exact sequence of a protein. If the recipes have extra bases or misspelled bases or if some are deleted, the cell can make a wrong protein or too much or too little of the right one. These mistakes often result in disease. In some cases, a single misplaced base is sufficient to cause a disease, such as sickle cell anemia.

Errors in our genes, our genetic material, are responsible for an estimated 3,000-4,000 hereditary diseases, including Huntington disease, cystic fibrosis, and Duchenne muscular dystrophy. What's more, altered genes are now known to play a part in cancer, heart disease, diabetes and many other common diseases. Genetic flaws increase a person's risk of developing these more common and complex disorders. The diseases themselves stem from interactions of such genetic predispositions and environmental factors, including diet and lifestyle. Some experts estimate that half of all people will develop a disease that has a genetic component.

Understanding the genetic code did not directly lead researchers to disease genes. Their ability to decipher the genetic messages encapsulated in DNA was stymied by the overwhelming number of such messages carried in the DNA of each cell. A human cell (except sex cells--sperm and egg cells--and some blood cells that have no nuclei) contains about 6 feet of DNA molecules tightly coiled and packed into 46 chromosomes--rod-like structures in the cell nucleus that are formed from DNA covered with proteins. This DNA is made up of 3 billion base pairs. If printed out, those base pairs would fill more than 1,000 Manhattan telephone directories. When researchers tried to break up DNA molecules into more manageable pieces, however, they ended up with a chaos of random fragments whose order in the original DNA was lost.