Persistent expression of the γ-globin genes in adults with deletion types of hereditary persistence of fetal hemoglobin (HPFH) is thought to be mediated by enhancer-like effects of DNA sequences at the 3′ breakpoints of the deletions. A transgenic mouse model of deletion-type HPFH was generated by using a DNA fragment containing both human γ-globin genes and HPFH-2 breakpoint DNA sequences linked to the core sequences of the locus control region (LCR) of the human β-globin gene cluster. Analysis of γ-globin expression in six HPFH transgenic lines demonstrated persistence of γ-globin mRNA and peptides in erythrocytes of adult HPFH transgenic mice. Analysis of the hemoglobin phenotype of adult HPFH transgenic animals by isoelectric focusing showed the presence of hybrid mouse α₂-human γ₂ tetramers as well as human γ₄ homotetramers (hemoglobin Bart’s). In contrast, correct developmental regulation of the γ-globin genes with essentially absent γ-globin gene expression in adult erythroid cells was observed in two control non-HPFH transgenic lines, consistent with autonomous silencing of normal human γ-globin expression in adult transgenic mice. Interestingly, marked preferential overexpression of the LCR-distal ^Aγ-globin gene but not of the LCR-proximal ^Gγ-globin gene was observed at all developmental stages in erythroid cells of HPFH-2 transgenic mice. These findings were also associated with the formation of a DNase I-hypersensitive site in the HPFH-2 breakpoint DNA of transgenic murine erythroid cells, as occurs in normal human erythroid cells in vivo. These results indicate that breakpoint DNA sequences in deletion-type HPFH-2 can modify the developmentally regulated expression of the γ-globin genes.