Tal-effector nucleases (TALENs) are engineered proteins that can stimulate 
precise genome editing through specific DNA double-strand breaks. Sickle cell 
disease and β-thalassemia are common genetic disorders caused by mutations in 
β-globin, and we engineered a pair of highly active TALENs that induce 
modification of 54% of human β-globin alleles near the site of the sickle 
mutation. These TALENS stimulate targeted integration of therapeutic, 
full-length beta-globin cDNA to the endogenous β-globin locus in 19% of cells 
prior to selection as quantified by single molecule real-time sequencing. We 
also developed highly active TALENs to human γ-globin, a pharmacologic target in 
sickle cell disease therapy. Using the β-globin and γ-globin TALENs, we 
generated cell lines that express GFP under the control of the endogenous 
β-globin promoter and tdTomato under the control of the endogenous γ-globin 
promoter. With these fluorescent reporter cell lines, we screened a library of 
small molecule compounds for their differential effect on the transcriptional 
activity of the endogenous β- and γ-globin genes and identified several that 
preferentially upregulate γ-globin expression.