SRC frequency evaluation in LDA. Summary of the level of human engraftment (y-axis) in the BM of mice given transplants with decreasing doses of CD34⁺ cells (x-axis) harvested at different time points of cultures after repeated expansion-isolation procedures over 20 weeks of culture, in LDAs. Each symbol represents a mouse. Flow cytometric analyses have been performed as described in “Patients, materials, and methods.” The presence of at least 0.1% of human CD45⁺, CD71⁺, and GpA⁺ cells in the BM of NOD/SCID mice plus the presence of a human colony in 500 000 plated unseparated bone marrow cells defined a positive engraftment. In addition all mice were to have detectable human lymphoid (CD34^-CD19/20⁺) and myeloid (CD45/71⁺CD15/66b⁺) engraftment (5 positive events each/20 000 assessed). The SRC frequency was calculated from the proportions of negative mice in each cohort, using L-Calc T software program (Stem Cell Technologies), which uses Poisson statistics and the method of maximum likelihood. The 95% confidence intervals were: input SRC frequency 1:35 270 (lower frequency 1:60 903; upper frequency 1:20 426), χ² (Pearson) P = .7406; week 4 SRC frequency 1:10 605 (lower frequency 1:18 487; upper frequency 1:6,083), χ² (Pearson) P = .3183; week 8 SRC frequency: 1:6995 (lower frequency 1:14 425; upper frequency 1:3353), χ² (Pearson) P = .88; week 12 SRC frequency 1:5263 (lower frequency 1:9216; upper frequency 1:3006); χ² (Pearson) P = .989; week 16 SRC frequency 1:1716 (lower frequency 1:3494; upper frequency 1:843), χ² (Pearson) P = .9826; week 20 SRC frequency 1:83 892 (lower frequency 1:161 649; upper frequency 1:43 538), χ² (Pearson) P = .001.