A diagram of the postulated effect of hydroxyurea in inhibiting hemoglobin S-polymerization, by increasing hemoglobin F levels (shown as 25%) in each sickle erythrocyte and thus decreasing the degree of microvascular obstruction at any oxygen level. The sparing effect of hemoglobin F, greater than that of hemoglobin A, occurs because the mixed hybrid α₂β^Sγ that forms inside the red cell does not enter the polymer phase. Adapted from Schechter and Rodgers (N Engl J Med. 1995; 334:333-335), with permission. Illustration by Alice Y. Chen.