Northern-blot hybridization, also referred to as Northern blotting, is one of several methods developed to detect the presence, to determine the size, and to quantify specific cellular mRNAs. By this method, total RNA or poly(A)⁺mRNA, prepared from the cells or tissue of interest, is fractionated by size on a denaturing agarose gel. The separated RNAs are transferred to a membrane by capillary action or under a vacuum and hybridized to a labeled probe with a base sequence complementary to all or part of the target mRNA. Analysis of hybridization signals determines whether the gene of interest is expressed in the cells or tissue, as well as the size and relative quantity of the target mRNA, if appropriate markers are used.