The locus control region of the beta-globin cluster contains five DNase I 
hypersensitive sites (5'HS1-5) required for locus activation. 5'HS3 contains six 
G-rich motifs that are essential for its activity. Members of a protein family, 
characterized by three zinc fingers highly homologous to those found in 
transcription factor Sp1, interact with these motifs. Because point mutagenesis 
cannot distinguish between family members, it is not known which protein 
activates 5'HS3. We show that the function of such closely related proteins can 
be distinguished in vivo by matching point mutations in 5'HS3 with amino acid 
changes in the zinc fingers of Sp1 and EKLF. Testing their activity in 
transgenic mice shows that EKLF is a direct activator of 5'HS3.