Induction of HbF levels by hydroxyurea. (A) Linear regression analysis indicates an inverse correlation between age and baseline HbF levels with 95% confidence interval (dotted lines). (B) Paired-sample analysis from individual subjects at baseline (□) and at MTD (Δ) illustrates the increase in HbF with hydroxyurea treatment per patient (gray lines) and mean ± SE (black lines) for each group of samples. (C) A histogram showing the variable HbF-induction in patient samples, with the absolute change in HbF (ΔHbF) between baseline and MTD (*P < .001 baseline HbF compared with MTD by the paired t test).