The hypothesis that bone marrow stem cells are a determining factor in ageing will be tested in a mouse model of premature ageing. First, the number and homing capacity of bone marrow stem cells from wild type p53+/+ and premature ageing p53+/m C57BL/6J mice will be assayed in vitro and in vivo. Then, both the ability of wild-type bone marrow stem cells to correct premature ageing in p53+/m mutant mice, and the ability of bone marrow stem cells from premature ageing p53+/m mutant mice to induce premature ageing, in wild-type recipients will be determined. In Specific Aim 1, the number of stem cells in wild-type p53+/+, p53+/- and p53+/m mice will be determined by flow cytometry and by in vitro colony formation, and homing determined by in vitro migration and marrow repopulation in vivo. In Specific Aim 2 bone marrow cells from GFP+ male C57BI6 p53+/+ mice will be transplanted to p53 mutant (p53+/m) and to wild-type (p53 +/+) female mice and the effect on the development of the aging phenotype determined. The contribution of donor cells to recipient tissues will be identified by the presence of GFP and the Y-chromosome. If ageing is due to a decreased ability of bone marrow stem cells to contribute to replacement of normal tissues in this model, transplantation of bone marrow cells from normal mice to p53 mutant mice should restore normal phenotype and normal life span. In Specific Aim 3 bone marrow cells from GFP+ p53+/m male mutant mice will be transplanted to irradiated normal wild-type female mice and to female mutant mice to determine if the aging phenotype can be transferred mutant bone marrow stem cells. Finally the possibility of fusion of donor bone marrow cells with tissue cells of recipient mice and the presence of fused cells in tumors will be determined. The results of these experiments should provide critical proof for one of the most important hypothetical properties of adult stem cells.