text: how is cancer more than 100 distinct diseases?   cancer is the result of an accumulation of many different mutations that cause cells to grow out of control.  skin cancer has a different set of mutations than brain cancer, for example, and so they are distinct at the molecular level.  not only that, but the cells themselves are often already differentiated, so skin cancer is really an unchecked growth of skin cells, whereas the brain cancer is an unchecked growth of brain cells, etc.