American Association for Cancer Research

Basic Biology and Cancer


How Cancer Begins

Where does cancer come from? It comes from normal tissues: Cancer cells are good cells that have gone bad. Cancer is often thought of as just one disease, but actually many, many different diseases make up what we think of as cancer. To date, more than 100 different cancer types have been identified.

How a cancer behaves biologically depends on the site in which it develops, and each cancer that develops in a specific area—the breast, the liver, the brain—has very distinguishing characteristics. We have different terms for different types of cancers. The word carcinoma, for example, means a cancer that starts in the epithelial cells, the cells that line the body’s organs. Carcinomas represent the vast majority of cancers. Other cancers begin in the mesodermal cells; these are the cells that make up the bones and muscles in the body. When a cancer develops in these tissues it’s called a sarcoma. There are cancers that develop in a glandular tissue, like the prostate; these are called adenocarcinomas. And there are leukemias, which come from the bone marrow cells that make blood, or the blood cells themselves, and lymphomas, which come from the lymph cells in the glands.