Quantitative competitive (qc) PCR. (A) Construction of an internal standard for competitive PCR. By amplifying the target template using a composite primer approach, a truncated product is obtained, with the same primer sites as the target. This is made possible by synthesizing a sense primer for an internal priming site, which also has the regular sense primer sequence added onto its 5′ end (composite primer). (B) Use of internal standard in qcPCR. Replicate PCRs are set up with a fixed aliquot of the unknown template. A dilution series of the competitive standard with known copy number is spiked into the PCRs. After amplification, the competitive PCRs are run on an agarose gel. Equivalence of PCR products (EQ) occurs when the target and standard templates were present in equal initial concentration, permitting quantification of the target template.