The key to this technique is obtaining a pure ribonucleic acid (RNA) sample. There are a number of ways to accomplish this depending on the size of the RNA, the level of purity you require and the equipment you have on hand. Once you have your RNA sample, mix it with a dye and place it into a sheet of electrophoresis agarose gel. Pass a current across the gel. The RNA will migrate across the gel sheet in a particular way that indicates its characteristics. Then place a nylon membrane on the gel and top it with an absorbent material (such as paper towels) and weight it. This is where the name "blot" comes from. The dyed RNA is fixed on the membrane.
To see the position of the various RNA pieces on the membrane, incubate the membrane with radioactive labels. Radioactive labels are chosen according to which particular section of RNA you are investigating. After the incubation period, place the membrane on a piece of X-ray film; the radioactive labels will expose the film, creating a picture of what is on the membrane. Once the film is developed, you have a picture of the relative position of the RNA sections you wish to examine.
Interpreting the results of the northern blot is a separate step. You can compare your RNA sample to other known RNA samples to identify and quantify the RNA you have. Cancer researchers often compare the RNA of cancer cells before and after chemotherapy treatment to determine the effects of chemotherapy medications on particular types of cancer. The northern blot test is relatively inexpensive, easy to perform and provides great utility in scientific research.