A Northern Blot is a method by which RNA is identified. After being subject to electrophoresis (usually gel electrophoresis) and being transferred to a filter, the RNA of interest is identified using fluorescent or radioactive probes which are short segments of DNA with a complementary sequence to the RNA of interest. The probed filter is "read" either by autoradiography - exposure to X-ray film, or by a phosphoimager. The size of the RNA is determined by the position of the band on the transferred filter (and is therefore independent of the probe size). A rough quantification of the amount of RNA present in the sample can be determined by comparing the amount of signal to that of a known control.
Although in wide use, it may one day be replaced with newer technology, such as real-time PCR.
The name has no geographical connotation. It is a play on words from Southern Blot, an essentially identical procedure used to detect DNA developed by the English scientist Ed Southern. There also exists a similar, but different method of detecting proteins called a Western Blot.