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A possible mechanism for enhancement of increased production of tumor angiogenic factor.

It is suggested that anoxic conditions may enhance the transformation of tumor cells to increase production of tumor angiogenic factor. The theory of oxygen diffusion into a spherical mass is used to calculate an upper bound for the critical size at which anoxia begins to develop in the center of an avascular clone of tumor cells. The predicted size is in line with experimental observations of the maximum size of avascular tumors.