Effects of aging and previous transplantation on the proliferation potential of CRUs. After determination of the CRU frequency in fresh and homed BM (Tables 1, 2), it was possible to retrospectively identify mice that had been injected with less than 0.3 CRU and in which the lymphoid and the myeloid compartments were subsequently repopulated with donor stem cells. On the basis of Poisson statistics, it is 95% probable that such mice were engrafted with a single HSC. Clone sizes generated by single, fresh (□) or homed (▨) CRUs as a function of donor and recipient age are represented as the mean ± SEM percentage of donor (Ly-5.2+)-derived PB leukocytes assessed 5 to 26 weeks after transplantation (4-8 mice per group from 2 pooled experiments). Note that data for the young-to-young group were collected contemporaneously with those for the other groups but were, in part, reported in Szilvassy et al.²³