HbF distribution in sickle erythrocytes from 3 patients with 20% HbF. An example of 3 possible distributions of HbF/F-cell in 1000 cells from 3 individuals with mean HbF levels of 20%. The y axis represents numbers of cells, and the x axis represents HbF concentration in 5-pg increments. In red are cells likely to be least protected from HbS polymer damage. These cells have HbF < 6 pg and are not visible by FACS. In yellow are cells visible by FACS but not fully protected from HbS polymerization. They have HbF concentrations of 6 to 10 pg. In green, are cells with HbF concentrations >10 pg. These cells should be fully protected from HbS polymerization. Many other distributions are possible with the same mean HbF accounting for the varied phenotypes associated with the same HbF concentration. Data are derived from Steinberg et al.⁵²