A 1.1 kb region of MBV genomic DNA that contains the 261 bp fragment was amplified and sequenced for each of the MBV isolates used in this study. The results demonstrated that the binding site of primers 261F/R is highly conserved among all geographic isolates studied (data not shown). Only one mismatch, located in primer 261F, was found in three isolates (01TH, 02MA1 and 04HA2), while the other isolates showed an exact match to the primer binding sites of the MBV genomic sequence.