EN
The probable role of stem cells has been presented in the development of tumors. It has been stressed that in contrast to immature, capable of proliferation and undifferentiated cells, the mature cells are unable to yield a neoplastic clone. The existence of cell clones or subclones determines the extent of differentiation in tumor cells and, thus, results in the heterogeneity of the tumor. Biological activity of such tumors involves production of several substances which negatively affect the organism of the host.