A risk factor is anything that changes your chance of getting a disease such as cancer. Different cancers have different risk factors. Lifestyle-related risks are thought to be the main factors that affect cancer risk cancers in adults. Examples include the effect of unhealthy diets (not eating enough fruits and vegetables, etc.), not enough exercise, and habits such as smoking and drinking alcohol. Lifestyle-related risk factors have little or no affect on childhood cancer.

Cancer is caused by a mutation (change) in a gene. During the past few years, scientists have made great progress in understanding how certain changes in a person's DNA can cause cells of the body to become cancer. DNA carries the instructions for nearly everything our cells do. We usually look like our parents because they are the source of our DNA. But DNA affects more than how we look. It also affects our risks for developing certain diseases, including some kinds of cancer. When children are born with mutated DNA that was inherited from parents, the mutations are present in every cell of the child's body. That means the mutations can be found by testing DNA of blood or other body cells.

Most cancers, though, are not caused by inherited DNA mutations. They are the result of DNA changes that happened early in the child's lifetime. Every time a cell prepares to divide into 2 new cells, it must copy its DNA. This process is not perfect, and errors sometimes occur. Luckily, cells have repair systems that "proofread" DNA. Some errors can still slip past, especially when the cells are growing quickly. This kind of gene mutation can happen at any time in life and is called an acquired mutation.

Acquired mutations start in one cell of the body, and that cell passes the mutation on to all the cells that come from it. These acquired mutations are present only in the patient's cancer cells and will not be passed on to his or her children. Although the causes of mutations responsible for certain adult cancers are known (for example, cancer-causing chemicals in cigarette smoke), the reasons for DNA changes that cause childhood cancers are not known. Some of these changes can take place in developing fetuses and are already present at birth.