To measure the size and amount of RNA transcribe from a specific gene of interest.
Northern blot first uses denaturing gel to separate RNA according to the size. The RNA is then transferred to a nylon membrane while keeping the same distribution in the gel. After fixing the RNA to the membrane, labeled probe complementary to the gene of interest is then added to hybridize to the immobilized RNA. The nonspecifically bound probes are then washed away. The solid membrane with probe specifically bound to RNA of interest is then dried, exposed and analyzed. Since northern blot uses size-dependent separation, this technique can not only determine the abundance but also the sizes of transcript of interest. It can be a very effective way to detect transcript variants of genes. However, if the amount of total RNA for the experiment is limited and expression level of transcript of interest is low, other techniques more sensitive than northern blot, such as quantitative RT-PCR, can be used.