People with Down Syndrome rarely get most kinds of cancer and U.S. researchers have nailed down one reason why — they have extra copies of a gene that helps keep tumors from feeding themselves.The findings could lead to new treatments for cancer, the researchers reported in the journal Nature on Wednesday, and further study of Down patients might reveal more ways to fight tumors.The researchers at Harvard University and elsewhere made use of a new kind of embryonic-like stem cell called an induced pluripotent stem cell or iPS cell. These cells, made from ordinary skin, can be transformed to act like powerful stem cells, the body's master cells.Using iPS cells from a volunteer with Down syndrome and mice genetically engineered to have a version of the condition, the researchers pinpointed one gene that protects against tumors."It is, perhaps, inspiring that the Down syndrome population provides us with new insight into mechanisms that regulate cancer growth," they wrote.Down syndrome is the most common genetic cause of mental retardation, occurring in 1 out of 700 live births.The Down syndrome theory had long been explored by Harvard's Dr. Judah Folkman, who died last year. Folkman, whose name is on the study, developed theories about how tumor cells grow blood vessels to nourish themselves in a process called angiogenesis.