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β-thalassemia major can be caused by homozygosity or compound heterozygosity for β-globin gene mutations (HBB gene). Most cases are inherited from parents who both have diseased alleles of the HBB gene. We report a patient with late-onset β-thalassemia major that evolved from β-thalassemia minor in which only one of her parents had the diseased HBB gene. To study the cause of β-thalassemia major in this patient, we performed the 100K single nucleotide polymorphism genotyping assay, fluorescence in situ hybridization, and DNA methylation analysis of the imprinting genes near the HBB gene. The results showed a loss of heterozygosity in the region of chromosome 11p14.3 to 11p15.5, which perfectly matched one allele of her father. Our study demonstrates that paternal uniparental isodisomy of chromosomal 11p15.5 is associated with the β-thalassemia major in this patient. Key words: β-thalassemia major, uniparental isodisomy, mosaicism.