I am pleased to include this book as Volume 6 of Developmental Biology: A Comprehensive Synthesis. It has been edited by two of the foremost investiga­ tors in the study of genomic adaptability. lowe a special debt of gratitude to Dr. Marie A. DiBerardino, who developed the concept of the volume. Dr. DiBerar­ dino is also a very active member of the editorial board for this series. Much of the success of this series is due to her valuable advice. This series was established to create comprehensive treatises on specific topics in developmental biology. Such volumes serve a useful role in developmental biology, since it is a very diverse field that receives contributions from a wide variety of disciplines. This series is a meeting ground for the various practitioners of this science, facilitating an integration of heterogeneous information on specific topics. Each volume is intended to provide the conceptual basis for a comprehensive understanding of its topic as well as analysis of the key experiments upon which that understanding is based. The specialist in any aspect of developmen­ tal biology should understand the experimental background of the field and be able to place that body of information in context to ascertain where additional research would be fruitful. At that point, the creative process generates new experiments. This series is intended to be a vital link in that process of learning and discovery.