The segments of the genes that contain the nucleotide variations (single nucleotide polymorphisms) of interest are amplified by PCR. PCR products are subjected to a multiplex primer extension reaction (PEXT) using allele-specific primers. Only the primer whose 3 end is fully complementary to the target is extended by the DNA polymerase. Biotin (circled B) is incorporated in the extended fragment. The extension product is directly applied to the dipstick-type biosensor.