"Somatic mosaicism is the presence of genetically distinct populations of somatic cells within an organism and even within the same tissue. Somatic rearrangements in various cancer cell types are well documented, and more than 30 Mendelian diseases have been associated with somatic mosaicism (13). Somatic mosaicism is well known to occur in normal germline cells as the process of meiotic recombination and in cells of the immune system within Ig and T-cell receptor genes as a result of V(D)J recombination that provides diversity in immune responses. Other studies have revealed age-related structural variation in human blood cells (14), aneuploidy and retrotransposition in the human brain (15, 16), and aneuploidy in human preimplantation embryos (17). Evidence of copy number variation (CNV) in monozygotic twins (18) also reveals the presence of somatic mosaicism in tissues arising from the same zygote."