Distributions of TPO thresholds for progenitors of 1-, 2-, 3-, and 4-cell MK clones and the total population. The bell-shaped curves are the gaussian analytical derivatives of the cumulative dose-response curves for 4 clone sizes and the total population. The peak of each curve indicates the median TPO threshold for the indicated class and corresponds to the respective TPO₅₀ shown in Figure 2. The solid line represents the TPO threshold distribution for the total MK progenitor population. The 95% confidence intervals (95% CIs) of TPO₅₀ were 2.68 to 9.04 pg/mL (total MK), 0.62 to 2.55 pg/mL (1 MK), 3.36 to 9.32 pg/mL (2 MKs), 13.9 to 21.7 pg/mL (3 MKs), and 32.3 to 35.7 pg/mL (4 MKs). Because the CIs did not overlap, TPO₅₀ values differed significantly from each other at the 5% level. A representative experiment is shown.