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The amount of amplified product is determined by the available substrates in the reaction, which becomes limiting as the reaction progresses.
When a person is in the early stages of infection, not a lot of virus is in the nose and throat, from which the samples are taken.
Specificity can be adjusted by experimental conditions so that no spurious products are generated.
It is used in and hybridization probing where amplification of only one of the two complementary strands is required.
Pitman Pura, New Delhi: New India Publishing Agency.