A test simply designated as “PCR” is a polymerase-chain-reaction test to detect DNA and is composed of 3 basic parts: 1) extraction of the DNA from the sample, 2) Addition of sample DNA, one set of DNA nucleotide primers and other reagents to a PCR-cycler machine for amplification of target DNA and, 3) Detection of target DNA by gel-electrophoresis. A test designated as RT-PCR is a reverse-transcriptase PCR test to detect RNA and is composed of the same 3 basic parts as PCR and an additional step using reverse-transcriptase enzyme to synthesize complementary DNA from the target RNA. The complementary DNA is then run in the PCR test. Nested PCR is a modification that uses 2 sets of nucleotide primers and 2 complete cycles of amplification; the second cycle of amplification further amplifies a target fragment of DNA originating within an already amplified larger target fragment of DNA. Nested PCR results in higher sensitivity than simple PCR or RT-PCR and is used for diseases that have very little target nucleic acid in tissue samples.