Analysis of Gene Expression by PCR

Methods used to study gene expression rely on two basic conditions. First, sufficient tissue must be available for analysis, and second, the gene must be expressed at a level high enough to be detectable by the method used. Where the gene of interest is expressed in small tissues or groups of cells, for example, in mouse embryos, or where transcripts are present in vanishingly low amounts, it is often difficult to perform Northern blotting, RNase protection assays, or in situ hybridization. In these cases, the exquisite sensitivity of the polymerase chain reaction (PCR) can provide a solution. PCR has been used to detect transcripts present at down to 1 copy/1000 cells (1 ), and in samples as small as a mouse blastocyst (2 –4 ) or even a single cell (5 ).