A novel beta-thalassemia mutation, not previously reported in the literature, was identified by direct DNA sequencing of the beta-globin gene. Hematological investigation of a 26-year-old woman due to her increased Hb A(2) level (6.2%) led to the identification of a heterozygosity for a 9bp (TCTGACTCT) deletion/insertion at codons 3-5. This was found to be the result of a deletion of cytosine (- C) at codon 5 (one of the nucleotides in the 13th or 14th position of exon 1), and an insertion of thymine (+ T) in front of codon 3 at the 10th nucleotide in exon 1 of the beta-globin gene. As a result of these mutations, the amino acids at codons 3-5 were changed from Leu-Thr-Pro to Ser-Asp-Ser. The whole frameshift was prevented by this rearrangement in the beta-globin gene. In addition, this result may provide important clues to identify critical amino acids responsible for stabilization of the hemoglobin tetramer.