Figure 2: Materno-fetal microchimerism established during pregnancy. During pregnancy, exchange of small numbers of maternal and fetal cells occurs by an unknown mechanism (possibly throughthe placenta), creating a chimeric state or microchimerism. Although the number of cells decreases after delivery, microchimeric cells are known to persist for the rest of our lives[1]. This suggests that various kinds of cells including stem cell-like cells withself-renewing potential might have migrated as microchimeric cells; however, little is known about their cell type repertoire.