The present invention relates to modified cancer cells that are defective in the production of diffusible factors (e.g., growth factors), and to the use of such cells for impairing the growth of a population of human cells, in particular wherein the cells are human cancer cells and the population of cells is a neoplasm, tumor or cancer. The invention preferably comprises the step of modifying tumor cells by deleting or modifying endogenous genes that code for (or affect the production of), diffusible growth factors that promote cell survival and proliferation.
