DNA molecules are in the nucleus of cells, also referred to as nuclear DNA. The depiction of DNA shows a double helix structure that resembles a spiral staircase. The basic unit of the DNA molecule is the nucleotide, which is composed of three components: a sugar, a chemical base that contains nitrogen, and phosphoric acid. The chemical bases provide the variation in each nucleotide unit, and the sugar and phosphorus portions form the "backbone" of the DNA molecule and do not change. A review of the history of methods for detecting DNA composition begins in the late 1980s with the complex multibanded pattern obtained by the Jeffreys' method and proceeds to early PCR (polymerase chain reaction) DNA testing. A description of current forensic DNA technology addresses STR (short tandem repeat) DNA analysis, as well as sources of DNA, including blood, semen, vaginal secretions, skin cells, and items that receive transfers of saliva and other bodily material and fluids. Also discussed is the interpretation of DNA results, the Combined DNA Index System (CODIS), and mitochondrial DNA analysis. 11 figures and 11 references