Expansion of CD34⁺CD38⁻ cells after transplantation of unfractionated or CD34⁺ cells. Data for 32 mice that showed detectable CD34⁺CD38⁻ cells from a group of 69 chimeric mice (>1% CD45⁺ cells). Twenty-eight mice were transplanted with unfractionated (unfract.) cells and 41 mice were transplanted with CD34⁺ grafts from 5 different UCB samples. The arrow shows the median expansion factor of CD34⁺CD38⁻ cells in each group.