One of the many lessons of the cancer cluster in Toms River, NJ - the subject of a new book, Toms River: A Story of Science and Salvation, by journalist Dan Fagin - is how hard it is to prove that one exists, or to pinpoint a cause. Proving that X chemical caused Y cancer is elusive. Worst of all is when you have disease and must search for a cause, said Trevor Penning, PhD, director of the Center for Excellence in Environmental Toxicity, in the Philadelphia Inquirer. "Then you begin to have an expensive conversation." One lesson of Toms River is that we need to know more about how carcinogens cause cancer, commented Timothy Rebbeck, PhD, professor of Epidemiology and the Cancer Epidemiology and Risk Reduction Program Leader in the Abramson Cancer Center.