Many situations arise in recombinant DNA research in which it is necessary to identify the mRNA encoded by a particular cloned DNA sequence. Cloned DNA fragments can be characterized by hybridization to mRNA, the complementary mRNA then being identified by translation in vitro. There are two different approaches to this procedure; hybrid-arrest translation (1) and hybrid-release translation (2). In the former, hybridization of cloned DNA to an mRNA population in solution can be used to identify the complementary mRNA, since the mRNA-DNA hybrid will not be translated in vitro (see Chapter 4 ). In the latter, described here, cloned DNA bound to a solid support is used to isolate the complementary mRNA, which can then be eluted and translated in vitro.