Variations in human genes responsible for detoxifying chemicals in the body can alter the extent of damage to DNA and affect human cancer risk. The goal of this study is to investigate the prevalence of genetic variations, called polymorphisms, in ten genes in two different groups of children, one with an elevated cancer risk, and the other with an increase in the occurrence of mutations, or changes in their DNA, in non-tumor cells, following chemotherapy. The first group was shown by the Environmental Protection Agency to have a 70% higher incidence of childhood cancer compared to the statewide incidence, as a consequence of drinking contaminated drinking water from hazardous waste sites in Toms River, New Jersey from 1979-1995. The second group was shown to have a 30-1300-fold increase in the occurrence of mutations at a marker gene, not involved in cancer, following chemotherapy treatment for leukemia. These studies will provide important information about the usefulness of studying polymorphisms in genes involved in chemical processing as markers of susceptibility both to childhood cancers and for acquiring mutations in children receiving chemotherapy.