Eighty percent of cancers, known as carcinomas, arise in epithelial cells—that is, the cells that make up the skin and the linings of organs. Breast cancers, for instance, don’t just grow randomly within the breast, but normally begin in the milk ducts. Epithelial cells are assumed to be particularly susceptible to cancers because they divide rapidly and often. Only about 1 percent of cancers are found in connective tissue; these are known as sarcomas.
