Reduced FOP expression correlates with high HbF in primary human cells. (A) Immunohistochemistry for γ-globin shows that more than 90% of FOP knockdown cells contribute to elevated HbF levels. (B) PB HEP cells express FOP at approximately 2.5 times higher level than FL cells. The level of FOP protein relative to actin; 3 FL and 3 PB donors were analyzed. *Significant difference (Mann-Whitney test, P < .05). (C) Reduced levels of FOP lower the expression of SOX6 but do not change the expression of BCL11A. (D) Quantitative S1 nuclease protection assay demonstrates an approximately 50% increase of γ-globin expression in erythroid progenitors from β-thalassemic patients when FOP levels are reduced.