Current concepts in biology of chromosomal mosaicism: somatic-germline aneuploidization pathway. Normal prenatal and postnatal development is hypothesized to be a matter of balance between two progressive processes: aneuploidization and "antianeuploidization" (the latter is arbitrarily covered by such term because it is still not completely clear what processes underlie the clearance of aneuploid cells in humans). Germline aneuploidzation results into prenatal death of aneuploid embryos or into chromosomal syndromes in newborns. Aneuploidization is observed in fetal germline tissues and in the fetal brain. This, if not cleared, has the potential to produce tissue-specific chromosomal mosaicism that can underlie the pathogenesis of brain diseases either in childhood or in adulthood. It also can be the reason of germline aneuploidization (mentioned earlier). Aneuploidization in adulthood (in some cases, in childhood) is suggested to be a key process of tumorigenesis and aging. This probably originates from the age-/environment-dependant inhibition of "antianeuploidization" processes.