The effect of the length difference between the negative strand and the positive strand (A), and the effect of the length of the probe (B) on the detection sensitivity and specificity of the displacing probe-based real-time PCR. Real-time PCR was carried out using the primer pair for IVS-2–654 site in the HBB gene with both wild-type and mutant templates. Water was used as the blank template. The probes of varied length used were based on the wild-type probe for the IVS-2–654 site. The probe was named according to the numbers of nucleotide of its positive (before) and negative strands (after), which were separated by a slash (e.g. probe 29/28, 5′-Fam-CTGGGTTAAGGCAATAGCAATATCTCTGC-PO4-3′/5′-CAGAGATATTGCTATTGC CTTAACCCAG-Dabcyl-3′, has a positive strand of 29 nt and a negative strand of 28 nt). The corresponding Tm values of both positive and negative strands are shown separately and are separated by a slash. The length change was made starting from the 3′-end of the positive strand, and accordingly from the 5′-end of the negative strands. Nucleotide changes were based on the original HBB sequence line.