In 1978 the Dutch Genetic SOciety organized a symposium on the genetic aspects of the origin of tumor cells. The objective of this symposium was to reach an overview of the state of knowledge in a number of quite different fields related to each other through the genetics of the initiation of tumor cells. This monograph contains the brought-up-to-date contributions of this symposium. Herein discussed is the extent that characteristics of tumor cells can be considered as a phenotype. The possible role of somatic mutation and repair of genetic damage is studied and the analysis of genes with oncogenic potential is pursued. Also the influence of host factors in the response to oncogenic action is dealt with. This volume describes in a clear and concise manner the current status in these research areas and, it is hoped, will stimulate the exchange of information and ideas between them. Dr. F.l. CLETON, The Netherlands Cancer Institute, Amsterdam Dr. J.W.I.M. SIMONS, Department of Radiation Genetics and Chemical Mutagenesis, University of Leiden CONTRIBUTORS P. Bentvelzen Ph.D. Radiobiological Institute TNO, Lange Kleiweg 151, Rijswijk (ZH), The Nether- lands. F.I. Cleton M.D. Antoni van Leeuwenhoek-Huis, The Netherlands Cancer Institute, Plesmanlaan 121, Amsterdam, The Netherlands. P. Demant M.D. Antoni van Leeuwenhoek-Huis, The Netherlands Cancer Institute, Plesmanlaan 121, Amsterdam, The Netherlands. A.I. van der Eb Ph.D., H. Iochemsen, I.H. Lupker, I. Maat, H. van Ormondt, P.1. Schrier.