It is now twenty years since Cohen and Boyer's first steps into DNA cloning. In the time since then, there has been an ever increasing acc- eration in the development and application of the cloning methodology. With the recent development of the polymerase chain reaction, a second generation of the technology has been born, enabling the isolation of DNA (and in particular, genes) with little more information than the p- tial knowledge of the sequence. In fact, DNA sequencing is now so advanced that it can almost be carried out on the industrial scale. As a consequence of these advances, it...