Pre-amplification scheme.

<p>(1) First strand cDNA synthesis is primed with tagged oligo-dT and random 9-mer primers. The tagged oligo-dT primer contains a VN anchor followed by a T-30 stretch with a 5′ PCR tag. The tagged random 9-mer consists of a 9-mer followed by the identical 5′ PCR tag. (2) Upon reaching the 5′ terminus of the mRNAs, the reverse transcriptase, via its terminal transferase activity, adds a few nucleotides (predominantly deoxycytidine) to the 3′ end of the newly synthesized cDNAs. (3) The template-switch primer, which consists of the same 5′ PCR tag as well as a 3′ riboguanine stretch, anneals via GC complimentary base-pairing to the 3′ end of the cDNAs, thereby serving as a new template for the reverse transcriptase. (4) After cDNA synthesis, both ends of the cDNAs now contain the identical PCR tag, allowing exponential amplification of the entire cDNA population through single primer PCR (5).