The Northern Blot is a technique used in molecular biology research to study gene expression. It takes its name from the similarity of the procedure to the Southern blot procedure (named for biologist Edward Southern ) used to study DNA, with the key difference that RNA, rather than DNA, is the substance being analyzed by electrophoresis and detection with a hybridization probe.

A notable difference in the procedure (as compared with the Southern blot) is the use of formaldehyde in the electrophoresis gel as a denaturant, because the sodium hydroxide treatment used in the Southern blot procedure would degrade the RNA.

As in the Southern blot, the hybridization probe may be made from DNA or RNA.

A variant of the procedure known as the Reverse Northern Blot was occasionally (although, infrequently) used. In this procedure, the substrate nucleic acid (that is affixed to the membrane) is a collection of isolated DNA fragments, and the probe is RNA extracted from a tissue and radioactively labelled.

The use of DNA microarrays that have come into widespread use in the late 1990s and early 2000s is more akin to the reverse procedure, in that they involve the use of isolated DNA fragments afixed to a substrate, and hybridization with a probe made from cellular RNA. Thus the reverse procedure, though originally uncommon, enabled the one-at-a-time study of gene expression using Northern analysis to evolved into gene expression profiling , in which many (possibly all) of the genes in an organism may have their expression monitored.