Recent studies have demonstrated that intravenous transplantation or local injection of bone marrow cells can induce unexpected changes of their fate. The results of these experiments showed that after transplantation or injecton, some of tissue specific somatic cells such as hepatocytes, skeleton, cardiac muscle cells and brain cells expressed the donor cell specific genes, such as Y chromosome. There are two hypotheses that can explain this phenomenon. One is bone marrow stem cell transdifferentiation and the other is spontaneous cell fusion. Recent studies have demonstrated that intravenous transplantation or local injection of bone marrow cells can induce unexpected changes of their fate. The results of these experiments showed that after transplantation or injecton, some of tissue specific somatic cells such as hepatocytes, skeleton, cardiac muscle cells and brain cells expressed the donor cell specific genes, such as Y chromosome. There are two hypotheses that can explain this phenomenon. One is bone marrow stem cell transdifferentiation and the other is spontaneous cell fusion.