The last decade has seen major advances in both cellular and molecular aspects of cancer biology. Tissue culture methods that allow the recognition of clonogenic cells within human tu mors have been described (37, 64); on the basis of theoretical arguments and the application of these techniques, it has been proposed that neoplasms may be regarded as stem cell systems in which a minority of cells have the proliferative capacity to maintain the tumor whereas the majority of cells demonstrate differentiation features and have limited proliferative potential.