HbF expression and silencing in human erythrocytes. (A) HPLC analyses were performed on peripheral blood of 86 infant donors and plotted as HbF/(HbA + HbF) (expressed as a ratio; y-axis) versus postnatal age (months; x-axis). (B) The distribution of HbF-silenced cells (expressed as a percentage; y-axis) versus postnatal age (months; x-axis) as determined by flow cytometry. The smooth trend lines were drawn using averages for each 10-week interval. The postnatal (PN) age of the donors is shown on the x-axis with vertical gray lines at 6 months. Postnatal ages range from 1 day to 60 months.