Chromosomal mosaicism is the presence of different karyotypes in two or more cell lineages within an individual derived from a single zygote1,2. This karyotypic variation may arise early in development and involve both the soma and the germline or it may occur later and be restricted to one or more specific cell types. In cancer, chromosomal anomalies can initiate a neoplastic clone or arise during clonal evolution and serve as clonal markers3. Here we consider such clonal variation as a form of mosaicism, since the cancer cells may have acquired one or more chromosomal abnormalities, while other cells in the same tissue, or elsewhere in the body, retain the normal karyotype. Chromosomal mosaicism in humans has been well studied in embryos4,5, fetuses from spontaneous abortions6, children with birth defects or developmental delay7,8 and cancer patients9. However, little is known about the type, frequency and age distribution of acquired chromosomal anomalies in large samples from the general population9,10.
