The powerful method of viral-induced fusion of animal cells was invented by Henry Harris and his colleagues in 1965, in order to study the genetics of somatic animal cells. This volume evaluates the impact of cell hybridisation on the study of cell differentiation, gene mapping, generegulation, and the development of monoclonal antibodies. Studies are presented on nuclear structure and function, intracellular transport, membrane protein mobility, and nuclear-cytoplasmic interaction in heterokaryons and other cells. Early experiments by Harris and co-workers suggested that geneloss plays an important role in tumour formation and that the malignant phenotype could be suppressed by hybridisation with non-malignant cells. This principle has since been shown to apply to a wide range of natural and experimental tumours, in species ranging from Drosophila to man.Tumour-suppressor genes are discussed, together with the role of radiation-hybrid mapping in the analysis of genetic tumours. The book concludes with an article in which Henry Harris examines unsolved questions that remain to be studied in the link between cell growth and differentiation.