It is clear that the tumor model, which has been used to explain tumor growth and to guide the design of new treatment schedules, needs to be improved. The study of tumor models requires the use of direct assays so that the validity of the hypothesis suggested by the model can be tested. The assays which have been used to study tumor stem cells are reviewed, and some preliminary results of attempts to study these cells in cell culture are presented. The existence of special classes of tumor stem cells, such as nonproliferating cells, and tumor stem cells which have an increased capacity to reproduce themselves and a reduced tendency to differentiate are postulated. The use of physical methods to study tumor cells is suggested as possible methods for detecting and studying special classes of tumor stem cells.

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This work was supported by a grant from the Ontario Cancer Treatment and Research Foundation.

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