Many people incorrectly believe that cancer is caused by one or two occurrences that transpire within the lifetime of an individual. This is not so. For a human being to acquire cancer, many factors are involved. Using phase 1(in vitro studies), phase 2(small animal studies), and phase 3(studies using humans) trials specific events have been identified as trigger factors for the incidence of tumors. However the individuals must have had a predisposition towards cancer to begin with. For example, in large pharmaceutical companies, the cell lines experimented with are usually derived from individuals that have specific treatable and untreatable cancer cells. The advantage to using cell lines which are carried from biopsies of cancer patients is many fold. These cell lines don’t die after 50 generations, as do cell lines derived from normal patients. Additionally, cancer cells don’t adhere as tightly to the petri dish in in vitro studies, and they round up, allowing for more cells to be fit on a plate than normal.