HbF induction in vitro by PHIs. HbF expression in erythroid progenitors that were differentiated from primary human CD34⁺ bone marrow–derived cells. The percentage increase in HbF mean fluorescence intensity is derived by comparison to cells stimulated with vehicle control (0.2% DMSO) without PHI or HU. PHI (20 μM) and HU (5 μM) were added as described in “Differentiation of human erythroid progenitors ex vivo.” Error bars are SE.