Disclosed herein is a polymerase chain reaction (PCR)-based method for detecting an insertion/deletion variant, as compared with a reference sequence, in a selected region of a target gene in a sample. The target gene has a template strand and a coding strand complementary to the template strand. The method uses a primer set that includes a blocking primer, and a forward primer. The blocking primer and the forward primer are partially overlapping and have different melting temperatures, and the 3′-end of the blocking primer is modified to prevent the extension of the blocking primer during the PCR. Accordingly, under specific PCR conditions, the presence of the PCR product is indicative of the presence of an insertion/deletion mutation in the selected region, and the absence of the PCR product is indicative of the absence of an insertion/deletion mutation in the selected region.