Fragile sites--points where chromosomes are apt to break--came to prominence in the late 1970s and are now the subject of much clinical concern and laboratory investigation. They are known or suspected agents in the genesis of mental retardation, birth defects, and cancer. This book, the first to deal with fragile sites on human chromosomes, takes a broad interdisciplinary approach to the subject, incorporating findings from cytogenetics, medical and clinical genetics, population genetics, mental retardation, dysmorphology, and oncology. The book has three interrelated sections. The first, focusing on the laboratory, presents what is known about fragile sites, their cytogenetics, and the conditions of cell culture needed to demonstrate them. A clinical section follows, covering X-linked forms of mental retardation, the clinical features associated with the fragile X, and genetic counseling with fragile sites. The final section considers fragile sites as they relate to genetics, including the possible relationship of fragile sites to cancer and to constitutional chromosome abnormalities. This book contains much previously unpublished material and will be an important resource for geneticists and cytogeneticists, molecular and cancer biologists, oncologists, pediatricians, and other health professionals.