At least two percent of people over age 40 and five percent of people over 70 have mutations linked to leukaemia and lymphoma in their blood cells according to research at the Washington University School of Medicine St. Louis. Mutations in the body's cells randomly accumulate as part of the ageing process and most are harmless. For some people genetic changes in blood cells can develop in genes that play roles in initiating leukaemia and lymphoma - types of blood cancer the researchers found. However the findings based on blood samples from nearly 3000 patients do not mean that people