Changes in F cells (A), MCV (B), neutrophil count (C), and reticulocyte count (D) during 2 years of treatment with HU according to the quartile of percent change in HbF at 2 years. Boxes indicate the 75th and 25th percentiles, which are connected at the median values. (•• — • •) fourth quartile; ( — • — ) third quartile; (––––) second quartile ( — — — — ); first quartile (……..).