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The probability of a colony which originated as a single stem cell to become extinct due to differentiation of all of its stem cells in any generation is closely connected to stem cell self renewal probability p . p can be determined from the coefficient of variation of the colony numbers received by reinjecting single colonies of the same age. Whole spleens containing a known average colony number can also be used with advantage for this purpose. The results of both procedures indicate a stem cell self renewal probability p = 0.62 ? 0.04, which does not change significantly between the sixth and the fourteenth day of colony development, and an extinction probability