In the United States, on average 1 to 2 out of 10,000 children develop cancer each year. The most common types of childhood cancer are leukemia (blood cell cancer) and brain cancers. Although no single cause of childhood cancer exists, some risk factors include, environmental factors, maternal medical conditions during pregnancy, maternal diet during pregnancy, fertility drugs, family history of childhood cancer or genetic susceptibility. A small percentage of cases are associated with Down syndrome, other specific chromosomal and genetic abnormalities. Childhood cancer is currently the leading cause of death by disease among U.S. children ages 1 to 14. Although the overall number of cases has increased over the past 30 years, mortality rates have decreased by more than half for childhood cancers.