If tumor viruses did not exist in nature they might have been created by scientists interested in basic mechanisms of develop- ment, differentiation, and tumorigenesis. In contemporary euka- ryotic cell biology tumor viruses playa similar role to that which bacteriophages once had for the molecular biology of prokary- otes. Tumor viruses provide extremely useful probes for the above cellular processes since their life cycle is genetically pro- grammed and can be followed at DNA, RNA, and protein levels. The experimental systems reviewed in this volume utilize a wide variety of viruses. A comprehensive introduction to this field has recently been published in the volumes of Molecular Biology o/Tumor Viruses: DNA Tumor Viruses, 2nd edition, edited by J. Tooze; and Molecular Biology o/Thmor Viruses: RNA Tumor Viruses, 2nd edition, edited by R. Weiss, N. Teich, H. Varmus, and J. Coffm, by Cold Spring Harbor Laboratories in 1980 and 1982. Polyoma and SV40 viruses (see the chapter by A. Levine) and adenoviruses (see the chapter by W. Doerfler) are double- stranded DNA-containing viruses. Polyoma and SV40 are struc- turally related viruses which contain a genome of approximately 5 kilo basepairs, while the DNA of adenovirus is about 7 times more complex. These DNA tumor viruses are understood at a genetic and molecular level which is comparable to our know- ledge of A and T4 bacteriophages. Retroviruses, the subject of the remaining four chapters, con- tain a single-stranded RNA genome of 5-8 kilobases.

During the past few decades we have witnessed an era of remarkable growth in the field of molecular biology. In 1950 very little was known of the chemical constitution of biological systems, the manner in which information was trans mitted from one organism to another, or the extent to which the chemical basis of life is unified. The picture today is dramatically different. We have an almost bewildering variety of information detailing many different aspects of life at the molecular level. These great advances have brought with them some breath-taking insights into the molecular mechanisms used by nature for rep licating, distributing and modifying biological information. We have learned a great deal about the chemical and physical nature of the macromolecular nucleic acids and proteins, and the manner in which carbohydrates, lipids and smaller molecules work together to provide the molecular setting of living sys tems. It might be said that these few decades have replaced a near vacuum of information with a very large surplus. It is in the context of this flood of information that this series of monographs on molecular biology has been organized. The idea is to bring together in one place, between the covers of one book, a concise assessment of the state of the subject in a well-defined field. This will enable the reader to get a sense of historical perspective-what is known about the field today-and a description of the frontiers of research where our knowledge is increasing steadily."