The polymerase chain reaction (PCR) is an in vitro method in which DNA sequences or transcripts are amplified rapidly with very high specificity and fidelity using oligonucleotide primers and Taq DNA polymerase in a simple automated reaction. Detection of viroids, viruses, bacteria, mycoplasma-like organisms, fungi, and nematodes by PCR has impacted diagnostic practices, epidemiological studies, as well as studies of pathogen-host-vector interactions. PCR provides a rapid, sensitive, and specific method for the detection of inserted recombinant viral or satellite viral DNA in the genomic DNA of transgenic plants. Templates chosen for use as internal standards in quantitative PCR fall into two categories: heterologous or "exogenous" templates and homologous templates that differ only slightly from target sequences. The use of homopolymeric tails in RACE may sometimes result in mispriming of the PCR and/or cDNA synthesis to generate a background population of nonspecific amplification products.