For the 43 individuals with hematological cancers for whom DNA was obtained at least a year prior to diagnosis, the frequency of detectable clonal mosaicism was 20% for myeloid leukemia and 22% for lymphocytic leukemia (predominantly chronic lymphocytic leukemia, Table 2) compared to 0.74% in 26,136 cancer free controls (overall OR=35.4, 14.7-76.6 95% CI, Fisher exact p=3.8×10−11). Of the 8 mosaic individuals with leukemia for whom DNA samples were collected at least a year prior to diagnosis, 4 were diagnosed with chronic lymphocytic leukemia (CLL) of which 2 had a mosaic deletion in a region of chromosome 13q14 previously described to be deleted in CLL20. DNA was obtained more than 5 years prior to diagnosis for 6 mosaic individuals, with the longest interval being 14 years, suggesting that detectable clonal mosaicism could be a marker of hematological cancer or its precursors, i.e., monoclonal B cell lymphocytosis (MBL) for CLL and myelodysplastic syndrome for acute myelogenous leukemia. Recent work shows that the majority of MBL have mono- or biallelic 13q14 abnormalities21. However, further studies will be needed, preferably with serial pre-
