Dose-response curves for CD34⁺CD41⁺ progenitors of megakaryocyte clones of various sizes. In the 5 top panels, dose-response curves have been fitted to the absolute frequencies of clones with the indicated numbers of megakaryocytes. In the bottom panel, the curves have been normalized to a plateau frequency of 100% and superimposed to facilitate comparison. Data points are the mean ± 1 SEM of quadruplicate determinations of the number of clones at day 5 per 96-well Terasaki plate. R² values for the gaussian cumulative model were 0.99 or more (P < .001) for all curves. 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. For 1 MK clones, the mean absolute number of clones per 96-well plate that survived in absence of TPO was 4.28 ± 0.72 (ie, 5.92% of the total clones). The dotted drop lines indicate TPO₅₀ (ie, the TPO dose at which 50% of the plateau number of clones were counted). Results are representative of 3 experiments.