R. Hass’s group has made essential contribution in the understanding of cell aging as works of that group have shown that metalloproteinase-7 and 20S proteasome directly participate in the signal transmission to PARP-1 protein which plays key role in hemopoietic cell rejuvenation. Moreover, experiments have shown that the decrease in the activity of metalloproteinases is linked directly to how rapidly cells become senescent, as well as to the tissue aging in general. At R. Hass’s laboratory, primary stem cells, obtained from the umbilical cord of newborns, are mainly used. Undoubtedly, this fact has great advantage in terms of subsequent use of the scientific data obtained for medical purposes. Particularly, the scientists from R. Hass’s laboratory have formulated a hypothesis about cell rejuvenation by means of their possible dedifferentiation.