Fragile sites common features of human (and also of other mammalian) chromosomes. Chromosomal fragile sites are non-randomly distributed loci that are especially prone to forming gaps, breaks or tri-radial figures in metaphase chromosomes when cells are cultured under conditions that inhibit DNA replication or repair. This new and important book is divided into 3 parts: 1. An overview about fragile sites on chromosomes, their classification, cytogenetics, expression, replication timing, and clinical significance and heritance, 2. The characteristics of fragile loci which lead to the expression of fragility specifically at these sites and 3. the link between fragile sites and human pathological conditions.