The reverse northern blot is a variant of a northern blot in which the nucleic acid immobilized on a membrane is a collection of isolated DNA fragments rather than RNA, and the probe is RNA extracted from a tissue and radioactively labelled.
DNA microarrays use methods akin to the reverse procedure, in that they involve the use of isolated immobilized DNA fragments, and hybridization with a probe made from cellular RNA. Thus the reverse northern blot, though originally uncommon, enabled northern analysis to evolve into gene expression profiling, in which many (possibly all) of the genes in an organism may have their expression monitored.