A risk factor  is anything that increases a person’s chance of developing cancer. Although risk factors often influence the development of cancer, most do not directly cause cancer. Some people with several risk factors never develop cancer, while others with no known risk factors do.
Although doctors don’t know what causes the vast majority of childhood leukemia, some evidence shows that certain genetic factors play a role in ALL. Children who are born with conditions that cause an abnormal immune system, such as Down syndrome, ataxia telangiectasia , and Bloom syndrome, may have a higher risk of developing leukemia. A child with an identical twin that develops ALL before age six has an increased risk of developing leukemia. If an identical twin develops leukemia within the first few months of life, the other twin will almost always develop the same type of leukemia.