In the mosaics described in this study, the chromosomally abnormal cells constitute a significant fraction of white blood cells, since a minimum of 5–10% is required for detection by our method and many abnormal clones are substantially larger (Figure 2). The blood samples used for DNA extraction were not fractionated by white blood cell type. The abnormal blood cells within an individual may include multiple cell types if the anomaly arose in a multipotent hematopoietic stem cell that became predominant due to senescence or positive selection within the stem cell population. Alternatively, the abnormal cells may include a restricted set of cell types, particularly when the normal composition of blood (i.e. 60–70% of neutrophils and 20–40% of lymphocytes39) is altered by unregulated proliferation40.
