Cumulative and derivative dose-response curves for the overall sample of clones derived from CD34⁺CD41⁺ progenitors. The cumulative dose-response curve is shown in the top panel, and derivative dose-response curve is shown in the bottom panel. Sorted progenitors were individually cultured for 5 days in serum-deprived medium with the indicated concentrations of TPO. Clones contained 1 to 16 megakaryocytes, corresponding to 0 to 4 progenitor doublings. Data points are the mean ± 1 SEM of quadruplicate determinations of the number of clones at day 5 per 96-well Terasaki plate. A gaussian cumulative equation gave the best fit to data among 29 functions based on transitional models (r² = 0.99; P < .001). The bell-shaped bottom curve, the gaussian analytical derivative of the top curve, is the frequency distribution of TPO thresholds of individual progenitors. Counts obtained for wells with no TPO were assigned a concentration of 10^–2 pg/mL to preclude entering the log of zero values in regressions. In this experiment, the mean absolute number of clones per 96-well plate which survived in absence of TPO was 4.44 ± 0.82 (ie, 6.05% of the total clones). The dotted vertical drop lines indicate TPO₅₀ (ie, the TPO dose at which 50% of the plateau number of clones were counted). Results are from experiment number 1 and are representative of 3 experiments.