In our view the central lesion in oncogenesis is the sustained, apparently uncontrolled proliferation of malignant cells beyond the regulated number of their normal counterparts. In general, therefore, the properties of cancer cells are most similar to those of the proliferating stem cells of that particular tissue or cell type. Thus in this view malignant cells do not differ in kind from normal cells, but mainly in the size of their proliferative compartments. Given this point of view, it is logical to assume that malignant cells utilize normal mitogenic pathways for their sustained proliferation and that one must look to means by which a particular class of cells controls its proliferation if one wishes to understand how malignancy is controlled in that cell type.