Isolation of the desired gene

Comparison of gene-expression patterns between cells and/or tissues facilitates the identification of molecules activated by a particular physiological or pharmacological treatment. The use of gene-expression profiling is particularly important in neuroscience, clinical science, stem cell biology, and metagenomic analysis. In many cases, however, the amount of specimen tissue available is limited, allowing only small amounts of mRNA to be obtained. As such, amplification of the isolated RNA is obligatory to obtain the microgram amounts of RNA required for microarray analysis or cDNA library preparation. Without amplification, such amounts of RNA would be obtainable only from millions of cells. Although the polymerase chain reaction (PCR) is a powerful method for amplifying a single target DNA, the exponential amplification that can be achieved using multiple targets (from mixtures of DNA fragments or mRNA molecules) often produces a biased sample, since cDNAs of differing lengths and composition are amplified with differing efficiencies.