As our understanding of human genetics has become more and more sophisticated, scientistshave developed increasingly powerful tools using genetic information to aid in identifying individualpeople. DNA can be helpful in many situations where identification is at question: For example,in criminal cases, forensic DNA evidence can link a suspect, or a weapon such as a knife, to a crimescene. It can also exclude a suspect. The best known sources of DNA evidence are blood andsemen, but increasingly it can be obtained from other items, such as a bottle cap, a toothbrush, a bitemark on a piece of cake, or a fragment of a contact lens. Another use is to identify human remains,such as of a soldier killed in battle. Or it might be used to determine biological relationship, suchas in paternity cases. This report focuses on using human DNA to determine individual identity, butanimal and plant DNA can also be used, to identify species (for example, did a sample of meat comefrom a protected whale species?) or even individuals (did a seed pod found in a truck come from aparticular tree at a crime scene?). (1) Those and similar developments raise several policy issues, including the use of genetic information in the criminal justice system, the impacts of continuing technological improvements,and the effects of the technology on privacy and individual rights. This report provides an overviewof how genetic information is used in identification and some of the issues associated with thoseuses. It begins by discussing the unique properties of genetic information that make it a powerfultool for identification and what is involved in making identifications from physical evidence. Nextis a description of current federal programs and activities related to DNA identification, followedby discussion of issues raised by the development of this new technology. Legislation activity in the106th Congress focused on several criminal-justice issues; for detailed discussion, see CRS Report RL30694(pdf) , DNA Evidence: Legislative Initiatives in the 106th Congress . The use of DNA in identification -- sometimes called DNA typing or DNA profiling (2) -- resultsfrom its unique characteristics. Those characteristics also affect how it can be used and the issuesthat arise from using it. Key features are described below. It is a complex molecule, containing much information. DNA is a chemical, deoxyribonucleic acid, consisting of modular components, called nucleotides, that are connected in a linear sequence. Each nucleotide contains one of four bases -- called adenine, cytosine, guanine, and thymine, oftendesignated by their initials A, C, G, and T. Each DNA molecule consists of two complementarystrands, with each adenine on one strand paired with a thymine on the other, and each guanine witha cytosine. The sequence of bases strung along a DNA molecule contains the information that formsthe basis of the genetic code of humans and most other organisms. (3) Much of the growing body ofknowledge about DNA comes from efforts of the Human Genome Project, a major goal of whichis to create a map of the code and identify the sequence of bases on the DNA molecule. (4) DNAidentification technologies tap a small part of the large set of information that the molecule contains. Each person has billions of identical copies of DNA in the cells of the body. Most of the billions of cells in a person's body contain identical sets of DNA molecules (5) -- approximately 3 billionbase-pairs per cell altogether. Most of the DNA is contained in 23 pairs of chromosomes in thenuclei of cells. Within each pair, one chromosome is inherited from the mother and the other fromthe father. Some DNA, inherited only from the mother, is also contained outside the cell nucleus,in structures called mitochondria, which are small bodies with many copies in each cell. Becausethe human body contains so many copies of DNA, even a very small amount of body fluids ortissues, such as blood or skin, can yield useful identifying information. The structure of the molecule, and therefore the information it contains, varies from person to person. DNA sequences from any two people will be the same at many points along the DNAmolecule, but the overall nuclear DNA sequence of each person is unique, except for identical twins,who have identical DNA. (6) On a single chromosome,individual alleles, which are discretecomponents of a DNA sequence, are inherited intact. Each pair of chromosomes has many pairs ofalleles (7) (the total number is as yet unknown), withone member of the pair inherited from the fatherand the other from the mother. The region of a pair of chromosomes containing a pair of alleles iscalled a locus. The DNA of the two alleles inherited at any particular locus may have identical ordifferent base-pair sequences. (8) It is the substantialvariation in DNA among people that ischaracterized with the technologies used in DNA identification. The structure is inherited, so the DNA of close relatives is more similar than that of distant relatives or unrelated people. Since half of a person's DNA comes from each parent, (9) full siblingsalso share half their alleles, on average, and grandparents and first cousins, one quarter. However,since a son receives his Y chromosome from his father, all the alleles on that chromosome areidentical to the father's and are inherited through the paternal line alone. (10) Similarly, all alleles inmitochondria are identical to the mother's and are inherited through the maternal line alone. (11) Therefore, the mitochondrial DNA of a male, for example, will be identical to that of his maternalgrandmother, and his Y-chromosome DNA identical to that of his paternal grandfather. Theinheritance patterns of DNA mean that analyzing a person's DNA can also provide identifyinginformation about a relative. Most of a person's DNA has no known biological function. Genes are segments of DNA thatcontain the code for making specific chemicals (mostly proteins). Humans have tens of thousandsof genes (the exact number is not yet known), but they comprise only a small part of a person'sDNA, most of which has no known function. However, many nonfunctional segments of a person'sDNA can be characterized with the techniques of molecular biology; those segments are called markers . The distinction between genes and markers has implications for use in identification thatwill be discussed later. DNA is easily preserved with the structure intact. DNA is a surprisingly stable chemical, and sequences can be easily preserved intact in dried or frozen samples. However, DNA can bedegraded, particularly in warm or moist environments or in the presence of many commonchemicals. The ability to use DNA in identification depends both on the size and the condition ofthe sample. That has consequences for sample collection, handling, and storage in applications suchas law enforcement. Using DNA in identification requires comparing DNA whose source has not been determined(such as from a crime scene or from a child in a paternity case) with DNA whose source is known(such as from a suspect or from a putative father). Four basic steps are involved: characterization,comparison, calculation, and interpretation. They are each discussed below, along with implicationsfor applications and issues. The first step is to characterize or profile corresponding DNA sequences in the samples to be compared. For forensic evidence in a criminal case, one or more samples of blood, semen, or othersources of DNA related to the crime will be processed, as will usually a sample from one or moresuspects. It is not possible with current technology to characterize the entire DNA sequence. Instead, alleles at specific loci are characterized. The technique used depends on the particular kindof marker. The major kinds of markers in use today are VNTRs, STRs, mtDNA, and certain simplersequences. For most of the fifteen years during which DNA typing has been used in forensics, VNTRs (variable number of tandem repeats) have been the major kind of marker used. They have thegreatest potential power to identify or exclude, because they vary more from person to person thanany other DNA system used in identification. They consist of sections of DNA in which a sequenceof about 15-70 bases is repeated many times. Analysis of a sample using VNTRs measures theapproximate number of repetitions in each marker examined. That number varies substantially fromperson to person. Analysis of VNTRs uses RFLP (12) technology, which requires that substantiallymore DNA be present, and in good condition, in the sample than do the other systems describedbelow. Therefore, many samples that are degraded or have very small amounts of DNA cannot beprocessed using VNTRs. STRs (short tandem repeats) also consist of repeated sequences, but the number of bases repeated is smaller (2-4), as is the total length of DNA comprising an allele (approximately 100-300bases for STRs versus 500-10,000 for VNTRs). STRs are not as variable as VNTRs, and thereforemore loci must be typed to obtain the same resolving power as with VNTRs. (13) Nevertheless, STRscan be examined with much smaller samples (for example, requiring a bloodstain the size of apinhead rather than one the size of a quarter coin), for two reasons. First, DNA-amplificationprocedures (known as PCR -- the polymerase chain reaction) can be used. The process uses theability of DNA to replicate itself to make many identical copies from a small initial amount of DNA. The procedure does not yet work with VNTRs. Second, because the sequences are shorter, they areless likely to be damaged if the sample is degraded. Therefore, such samples can often yield moreusable results with STRs than with VNTRs. STRs also have some other technical advantages overVNTRs. In particular, they can be processed in the laboratory much more quickly and they are easierto interpret.