In this rapidly advancing field there have been many discoveries in the six years since the second edition was written, so the time is ripe for a new edition to incorporate some of this new information. In order to keep the book within a reasonable length, and make it accessible to students living on ever decreasing grants, some material and topics of the earlier editions have been omitted or drastically shortened. The new edition has expanded coverage of both prokaryotic and eukaryotic replication and transcription, and post-transcriptional modifications of RNA. Elsewhere, other topics have been brought up to date. Inevitably, their selection has been largely influenced by my own interests and expertise. As there are several excellent accounts of genetic engineering available, Chapters 3 and 4 of the previous edition on methodology and vectors have been combined and shortened.

I am most grateful to Dr Tim Benton at CUP for his encouragement and gentle guidance; to Mrs Sarah Price for her meticulous and helpful copy editing; and, as always, to my wife for her support and also for help in the preparation of some of the figures. I have also been greatly helped by being allowed ready access to the libraries of the University of Hertfordshire and of my old College, the Medical College of St Bartholomew's Hospital.