Northern blots detect a target RNA sequences through the use of hybridization probes. RNA fragments are separated by size using gel electrophoresis. The sample is then transferred to a membrane using a buffer and capillary action. An RNA or cDNA probe of complementary sequence is used to probe the membrane where is binds to the targeted RNA sequence. Detection depends on the probe label but radioactive, chemiluminescent and fluorescence are commonly used. Northern blots are used in the analysis of gene expression.