Figure 1. Complementary deoxyribonucleic acid (cDNA) selection. The target DNA is labeled with biotin and repetitive DNA removed by a prehybridization step. This leaves nonrepetitive, biotin-labeled target DNA single stranded, which is then annealed to cDNA amplified up from an appropriate library, using vector-primed polymerase chain reaction (PCR). Streptavidin magnetic beads have a high affinity for biotin and can be used simply to isolate all the DNA that contains biotin. PCR amplification with the cDNA vector primers will amplify only cDNA that is homologous to the target DNA – all other cDNA is lost, because it has no biotin. The process can be repeated for three or more cycles.

Figure 2. Exon trapping. (a) The exon-trapping vector, with inserted target DNA that contains no exons, is expressed in the appropriate human cells and its ribonucleic acid (RNA) spliced to the vector donor (D) and acceptor (A) sites; reverse transcriptase (RT)-PCR of messenger RNA (mRNA) yields no product, because the inserted DNA transcript is spliced out. (b) If a fragment of DNA containing an exon is inserted into the vector, a new splice pattern occurs. RT-PCR of the cellular mRNA will result in a PCR product that contains the inserted target DNA exon. This can be sequenced or cloned as required.