The dust-jacket aptly sums up this study by Sir Henry Harris, Emeritus Regius Professor of Medicine and former Head of the Sir William Dunn School of Pathology at Oxford, as "the first scholarly history of research on the genetics of somatic cells". This is an interesting, elegantly presented, and certainly detailed, account of what might be called the flip side of the usual, well-worn story of the development of molecular biology during the twentieth century. Here, as an entirely different perspective from the now familiar, orthodox, "double helix" version of the origins of molecular biology out of classical (i.e. transmission) genetics, is an account of how genetic mechanisms have been approached through their translational effects in somatic cells, that is through the manifestations of genetics in the differentiation of cells. This alternative history ought, by rights, to be just as interesting and important as the orthodox one, but turns out to have been almost impossibly complex. The study of the genetics of somatic cells has been inseparable from an understanding of innumerable incidental biological issues, such as cell theory, the nature of mitosis, and the developmental properties of cells, but it has perhaps always had as its ultimate aim the understanding of cancer. As Professor Harris's account makes very clear, progress has again and again been determined by technical limitations and advances, including those of tissue culture, cytogenetics, cell cloning, establishment of markers of cellular differentiation, human gene mapping and cell fusion. The latter discovery seems to have been a key development in the field, needed, for example, to reveal tumour suppressor genes and map human genes. Some of these techniques themselves led on to important specific practical spins-off such as monoclonal antibodies. But a comprehensive understanding depended on a complex of ideas and only recently could the claim be made that some sort of natural culmination had been reached in the concepts of cell transformation, oncogenes, and more especially tumour suppressor genes. And now it seems that the field has finally established bridges with the techniques derived from transmission genetics and established a merger on the common ground of the recombinant DNA technologies, the human genome project and the growing interest in the evolutionary homologies of gene molecular sequences across species. Moreover, as Professor Harris finally concludes: "The two great peaks that somatic cell geneticists have long been attempting to scale, cancer and differentiation, seem to have merged into one". As history this book is unashamedly internalist. Much of it essentially tracks subjects in which Professor Harris has himself been a participant, through a career, one is reminded, of quite remarkable breadth. Even so the account is admirably dispassionate and fairminded. The style is dry and the technique is that of a scientific review, in that there is little comment, interpretation or analysis, other than the occasional expression of surprise that things happened the way they did. There is hardly any reference to the literature by other historians. It is in effect a very detailed and fully referenced listing of events and sometimes comes across as a catalogue of "firsts". The book contains fifty-six portraits of scientists, while another nine illustrations are taken from scientific papers themselves. The book lays down something of a challenge to historians, although it is far from clear whether it is intended as such. Professor Harris introduces it by saying: "It is difficult to see ... how someone who is not intimately familiar with the ideas and the technology that underlie a scientific discipline can write perceptively about it". Although this claim is unlikely to engage the immediate sympathy of