Cells differentiating toward either B or NM lineages undergo rapid and simultaneous acceleration of their proliferative activity. (A) Design of the cell division tracking strategy used to analyze the progeny of individual CD34⁺RA^–C^– cells. (B) Number of output cells per 1000 input CD34⁺RA^–C^– cells tracked over an 8-day period in the design shown in panel A (data points show the mean ± SEM values pooled from 5 experiments). (C) CFSE and surface marker expression profiles of cells obtained at different times of cultures initiated with CFSE-labeled CD34⁺RA^–C^– cells. The number of completed divisions was determined by the fold dilution of CFSE fluorescence intensity. (D) Numbers of the 3 output phenotypes detected in successive CFSE fluorescence peaks indicative of completion of different numbers of cell divisions (generated per 1000 CD34⁺RA^–C^– input cells; data points show the mean ± SEM values pooled from 5 experiments). (E) Average cell cycle transit times (in hours) preceding the appearance of different output phenotypes at different time points (data points show the mean ± SEM values pooled from 5 experiments). (F) Average cell cycle transit times (mean ± SEM) of the immediate progeny of different sources of CFSE-labeled CD34⁺RA^–C^–, CD34⁺RA⁺C^–, and CD34⁺C⁺ phenotypes assessed after a 4-day culture in LEM. Input cells tested were isolated directly from unmanipulated CD34⁺ CB cells (n = 3 experiments), xenografts (n = 4 experiments), and cultures (n = 7 experiments). ∗P < .05; ∗∗P < .01 (via pairwise t tests post Holm adjustment). (G) Assignment of G0, G1, and S/G2/M phases based on the intensity of Ki67 (log10-transformed pixel values) and DAPI (10⁵ pixel values) measured by immunofluorescence. Data shown are representative profiles of primary CB CD34⁺38^– cells (left) and pooled week 2 outputs of CB P-mix cells (right). (H) Proportions of week 2 output cells of CB P-mix at each cell cycle phase within each phenotypic population (bars showing the mean ± SEM values of 3 experiments). ∗P < .05, determined by t tests post Holm adjustment. DAPI, 4′,6-diamidino-2-phenylindole.