Reverse dot blot is featured by hybridization of labeled target DNA with immobilized probes. In conventional dot blot, each probe requires a separate hybridization, while in reverse dot blot all known allelic variants at several loci in a sample can be screened simultaneously. PCR was used to generate tandem polymers of ASO sequences for three β-thalassemia mutations [-28 (A→ G), CD17 (A→T) and CD41～42 (-TTCT) ], and then inserted the amplified probe polymers into plasmid. The final probes, which were amplified by PCR, were applied to the nylon membrane and hybridized with radioactive labeled HBB PCR products to detect the genotype of sample DNA.