The ability to fractionate nucleic acids and to determine which of them has sequences complementary to an array of DNA or RNA molecules is one of the most powerful tools of molecular biology. The Southern blot, named for its inventor, is a method for transferring size-fractionated DNA from a gel matrix to a solid support followed by hybridization to a labeled probe (1 . The identical process for RNA became known as) the Northern blot i2 1i The identical process for RNA became known as the Northern blot (2 ). Both are, then, often key elements in establishing the identity of nucleic acids of interest. Northern blot analysis was used by us as a tool in order to answer the question whether or not the nuclear transcripitional apparatus of human ovarian cancer cells might be activated in response to the urokinase.