RT-PCR (Section on Developments of the PCR technique) can be used to estimate gene expression. DNA is transcribed into RNA and then translated into protein. So the amount of RNA present is a measure of the gene expression. Isolated RNA is reverse transcribed into DNA (complementary DNA, known as cDNA), and then the gene amplified by PCR. In most cases the more RNA the more copies of the gene will be generated. When the PCR products are run on an agarose gel (see Figure 3 above) a band, corresponding to a gene, is seen on the gel. The more original template the wider and brighter the band seems. By running samples amplified from different tissues or from organisms treated in a variety of ways a general idea of the relative levels of gene expression is obtained. A quantitative method, QRT-PCR, has been developed.