Characterization ofβ-thalassemia mutations were attempted for 29 Japanese families clinically diagnosed as havingβ-thalassemia. Following the identification of a mutation by cloning and sequencing, all families were screened for this particular mutation, using biotinylated allele-specific oligonucleotide probes. Seven different mutations were detected in 17 families: Six families had the frameshift mutation at codons 41/42, resulting from a 4 nucleotide deletion (TTCTTT+- - - - TT); four had the deletion at codons 127/128 (CA66CT+CCT); and three had theTATAbox mutation at nucleotide -31 (A→6). Four additional families had mutations at codon 24 (GGT→GGA), codon 26 (GAG→AAG), IVS-11-654 (C→T) and codon 110 (GTG→CCG), respectively. The newly discovered deletion mutation at codons 127/128, and mutations at nucleotide -31, and at codon 110 are peculiar to Japanese, and have not been found in any other ethnic group. The haplotypes of theβ-globin gene cluster were also determined. Some of the haplotypes andβ-thalassemia mutations are identical to those reported in the Chinese population. However, it is noteworthy that nearly half of theβ-thalassemia mutations were unique to Japanese.