The persistence of fetal hemoglobin in many patients with deletion type beta 
thalassemias and the expression patterns of human globin genes in transgenic 
mice suggest that gamma- to beta-globin gene switching results primarily from 
competition of gamma- and beta-globin genes for interaction with the beta-globin 
locus control region (LCR). To define regulatory sequences that are essential 
for the competitive advantage of the gamma gene at early developmental stages, 
stable transgenic mouse lines were produced with LCR gamma-beta constructs 
containing deletions of gamma 5'-flanking DNA. All constructs contained the full 
22 kb LCR, a 4.1 kb beta-globin gene and a gamma-globin gene with 1348, 383, 
202, 130, 72 or 52 bp of 5'-flanking sequence. Primer extension analysis of yolk 
sac, fetal liver and blood RNA from these lines demonstrated that a region 
between -202 and -130 of the human gamma-globin gene promoter was required to 
suppress beta-globin gene expression at early developmental stages. Four 
transcription factor binding sites within this region [GATA(p), Oct1, GATA(d) 
and CACCC] were mutated independently in LCR gamma-beta constructs and 
transgenic mouse lines were produced. Only the gamma CACCC box mutation resulted 
in high levels of beta-globin gene expression in early embryos. These results 
demonstrate that the CACCC box of the human gamma-globin gene plays a critical 
role in human beta-globin gene developmental specificity. The data also suggest 
that gamma CACCC box binding factors mediate LCR-gamma interactions which 
normally enhance gamma-globin and suppress beta-globin gene expression in fetal 
erythroid cells.