Thailand deletion of alpha-Thalassemia (thal) 1 involves the zeta2-, phi zeta1-, 
alpha2-, alpha1-, and theta1-globin genes. In Southeast Asians and Taiwanese, 
this mutation is the second most common long-segment deletion of two 
alpha-globin genes, after the Southeast Asian deletion. To define the Thailand 
deletion breakpoints, we used polymerase chain reaction (PCR) to amplify the 
normal-sequence DNA fragments across the breakpoints. The amplified products 
were sequenced directly or after cloning into pGem-3Z or pCR2.1 vectors. 
Comparison of the normal and mutant sequences revealed that the 5' breakpoint 
lies between nucleotides 1,269 and 1,290 upstream of the initiator codon adenine 
of the zeta2-globin gene, and the 3' breakpoint lies between nucleotides 29,387 
and 29,408 downstream of it. A total of 30,677 nucleotides were deleted. Both 
breakpoints mentioned above lie within the Alu repetitive sequences and an 
extensive sequence homology is present around the two breakpoints. These 
findings suggest that homologous recombination is the mechanism by which the 
deletion occurs. Based on our data, we used three oligonucleotide primers to 
amplify the regions across the deletion and its corresponding normal sequence. 
The feasibility of PCR diagnosis was confirmed in 20 carriers with this 
deletion.