Many people are diagnosed with cancer and wonder: Why me? U.S. scientists said on Jan. 1 that as many as two-thirds of cancer patients may be blamed on “bad luck,” a random mutation that occurred when their healthy stem cells divided, while another third can be attributed to genetic and environmental factors. This finding will help researchers design more effective prevention strategies for different types of cancer. Johns? Hopkins researchers report in the latest issue of the journal Science that cancer occurs due to random errors, or mutations, in tissue stem cells as they replicate their DNA (deoxyribonucleic acid) during division, and the more mutations that accumulate, the greater the risk that the cells will become cancerous. To understand the magnitude of the effect of mutations, environmental factors and genetics on cancer during stem cell division, they analyzed published data on stem cell division in 31 human tissues and compared it to the incidence of cancer in these tissues. The study showed that the correlation between the number of normal stem cells dividing in a human tissue and the incidence of cancer in that tissue was 0.804, or highly correlated. According to a new statistical model developed by the researchers, the cancer incidence of a tissue is the square of this correlation, expressed as a percentage, which is 65 percent. As an example, the researchers say that human colon tissue has four times more stem cell divisions than small intestine tissue. Similarly, colon cancer is much more common in humans than in the small intestine. In mice, on the contrary, colon stem cells divide less frequently than small intestine tissue. Similarly, the incidence of colon cancer in mice was lower than that of small intestine cancer. Further analysis showed that 22 of the 31 tissues mentioned above could be largely explained by “bad luck” in stem cell division, including pancreatic, bone, ovarian and brain cancers, while the incidence of the other 9 cancers was higher than predicted by “bad luck”. The other nine cancers are more common than predicted by “bad luck. The researchers said the nine cancers may also take into account environmental or genetic factors, such as smoking for lung cancer and sun exposure for skin cancer. The researchers said lifestyle and habit changes can be a huge help in preventing specific cancers, but may not be as effective in preventing some others, so “we should devote more resources to diagnosing cancer at an early, treatable stage. The researchers emphasized that the study did not include breast cancer, the most common cancer in women, or prostate cancer, a common cancer in men, because they were unable to obtain reliable data on the number of stem cell divisions in these cancers.