Today, most crime labs use a DNA typing system that takes advantage of the polymerase chain reaction (PCR), a technique that duplicates short segments of DNA. PCR is used to make copies of the variable regions in human DNA to create a DNA profile. PCR enables DNA analysis with biological samples from dental molds, cigarette butts, beverage cans, eating utensils, chewing gum, postage stamps, ski masks, licked envelopes, toothbrushes, razor shavings, band aids, and clothing. DNA typing results are expressed as probabilities, which indicate the likelihood that a person chosen at random from a certain population can provide the DNA profile of the sample obtained from the crime scene. Other types of forensic DNA techniques described in this article are analysis of single nucleotide polymorphisms (SNP's) and mitochondrial DNA (mtDNA). The U.S. forensic science community has standardized the 13 short tandem repeats (STR's) series of human chromosomes into the FBI's national database structure, the Combined Index System (CODIS) Program. The CODIS program's three-tier structure allows State and local agencies to operate DNA databases according to their specific legislative or legal requirements. Courts have recognized the validity of the underlying theory and technique of DNA typing; however, the quality and integrity of particular DNA profile evidence must always withstand judicial inspection and challenges by defense attorneys. Providing a solid foundation for DNA evidence begins with local law enforcement practices. Dr. Paul B. Ferrara, Director of the Virginia Division of Forensic Science, suggests three practices that police managers should implement in providing admissible DNA evidence. These are to train all crime-scene personnel in the collection and preservation of DNA evidence, to accredit the labs that process their DNA evidence, and to fully fund and staff forensic labs and facilities.