The cause of most types of leukemia is not known. Exposure to radiation, to some types of chemotherapy, or to certain chemicals (such as benzene, some pesticides, and chemicals in tobacco smoke) increases the risk of developing some types of leukemia, although leukemia develops only in a very small number of exposed people. Certain hereditary disorders, such as Down syndrome Down Syndrome (Trisomy 21) and Fanconi anemia, increase the risk as well. In some people, leukemia is caused by certain abnormalities of the chromosomes.