The rate of change in the field of the new human genetics appears hectic. Each week brings reports of the localisation and cloning of new genes. The accelerating rate of change is well illustrated by the history of research on hereditary breast and ovarian cancer. Although it has been long accepted that a family history of breast cancer indicates some increase in risk for all women in the family, it was generally assumed that this was the result of many genetic and environmental factors working together. Few people believed until very recently that it was likely that single-gene effects were significant. In fact, as long ago as 1866, Broca, in his Traité des Tumeurs, published his wife's family tree showing ten women in four generations who had died of the disease. While today this pedigree would be interpreted as evidence of an autosomal dominantly inherited susceptibility, at the time its significance was not understood. It was not until 1990 that Mary-Claire King and her colleagues used genetic linkage analysis to identify a location on chromosome 17q for a gene that was named BRCA1. This observation was soon confirmed and extended to include ovarian cancer. By the time work had begun on this book, linkage testing for BRCA1 was just starting to be offered to members of suitable families. Indeed, one of the personal accounts in the book describes what may well have been the first occasion on which the results of linkage testing were used in making a decision about treatment.