The Full form of PCR is Polymerase Chain Reaction. PCR is a technique that is used to amplify trace amounts of DNA (and in some instances, RNA) located in or on almost any liquid or surface where DNA strands may be deposited. The key to understanding PCR is to know that every human, plant, animal, bacterium, parasite, or virus contains genetic material such as DNA (or RNA) sequences (nucleotide sequences or pieces of DNA or RNA) that are unique to their species, and to the individual member of that species. Consequently, if a sample contains segments of DNA or RNA, Polymerase Chain Reaction is a method used to amplify (make many more identical copies) of these unique sequences so they can then be used to determine with a very high probability the identity of the source (a specific person, animal, or pathogenic organism) of the trace DNA or RNA found in or on almost any sample of material. The PCR test forms the basis of a number of tests that help physicians diagnose and treat patients. For example, PCR tests can identify and detect pathogenic organisms in patients, especially those that are difficult to cultivate (for example, HIV and certain fungi and other viruses).