Structural chromosome abnormalities are relatively frequent in human populations. They are the result of breaks that disrupt the continuity of one or more chromosomes. Chromosome breaks in the germline can lead to heritable structural abnormalities; those occurring in somatic cells may increase the risk of cancer. Chromosomes may break at almost any point, but there are sites of preferred breakage, called hotspots. The breaks may be repaired, but because any two broken ends that are sufficiently close together in the nucleus may rejoin, an extremely wide variety of structurally altered chromosomes occur. The main classes of structural abnormalities are described in this and the next two chapters, and some of the resulting phenotypes, or clinical syndromes, in Chapters 15–20.