AIM The hepatitis B surface antigen(HBsAg)is consideredto be one of the best markers for the diagnosis of acute andchronic HBV infection.But in some patients,this antigencannot be detected by routine serological assays despitethe presence of virus.One of the most important explanationsfor the lack of detectable HBsAg is that mutations whichoccur within the“a”determinant of HBV S gene can alterexpression of HBsAg and lead to changes of antigenicityand immunogenicity of HBsAg accordingly.As a result,thesemutants cannot be detected by diagnosis assays.Thus,it isessential to find out specific and sensitive methods to testthe new mutants and further investigate their distribution.This study is to establish a method to investigate thedistribution of the HBsAg mutant at nt551.METHODS:A mutation specific polymerase chain reaction(msPCR)was established for amplifying HBV DNA with amutation at nt551.Four sets of primer pairs,P551A-PPS,P551G-PPS,P551C-PPS and P551T-PPS,with the samesequences except for one base at 3’terminus were designedand synthesized according to the known HBV genomesequences and the popular HBV subtypes,adr and adw,inChina.At the basis of regular PCR method,we explored thespecific conditions for amplifying HBV DNAs with a mutationat nt551 by regulating annealing temperature and theconcentration of these primers.126 serum samples frompatients of hepatitis B were collected,among which 16 werepositive for HBV S DNA in the nested PCR amplification.These 16 HBV S DNAs were detected by using the msPCRmethod.RESULTS:When the annealing temperature was raised to71 ℃,nt551A and nt551G were amplified specifically byP551A-PPS and P551G-PPS;At 72 ℃ and 5 pmole of theprimers(each)in reaction of 25 μl volume,nt551C andnt551T were amplified specifically by P551C-PPS and P551T-PPS.16 of HBV S gene fragments were characterized byusing this method.14 of them were positive for nt551A,one was positive for nt551G,and the other one was positivefor nt551T.The results were confirmed by nucleotidesequencing CONCLUSION: The mutation specific polymerase chain reaction is a specific and sensitive method for detecting the mutations of HBV genome at nt551. AIM The hepatitis B surface antigen (HBsAg) is considered to be one of the best markers for the diagnosis of acute and chronic HBV infection. But in some patients, this antigencannot be detected by routine serological assays despitethe presence of virus. One of the most important explanationsfor the lack of detectable HBsAg is that mutations whichoccur within the “a” determinant of HBV S gene can alterexpression of HBsAg and lead to changes of antigenicity and immunogenicity of HBsAg accordingly. As a result, these mutants can not be detected by diagnostic assays. Thus, it is essential to find out specific and sensitive methods to test the new mutants and further investigate their distribution. This study is to establish a method to investigate the distribution of the HBsAg mutant at nt 551. METHODS: A mutation specific polymerase chain reaction (msPCR) was established for amplifying HBV DNA with amutation at nt 551. Flexible sets of primer pairs, P551A-PPS, P551G-PPS, P551C-PPS and P551T-PPS, with the same sequence ex cept for one base at 3’terminus were designed and synthesized according to the known HBV genome sequences and the popular HBV subtypes, adr and adw, in China. At the basis of regular PCR method, we explored the specific conditions for amplifying HBV DNAs with a mutation at nt551 by regulating annealing temperature and the concentration of these primers.126 serum samples frompatients of hepatitis B were collected, among which 16 were positive for HBV S DNA in the nested PCR amplification.These 16 HBV S DNAs were detected by using the msPCRmethod.RESULTS: When the annealing At 72 ° C and 5 pmole of the primers (each) in reaction of 25 μl volume, nt551C and nt551T were amplified specifically by P551C-PPS and P551G-PPS; PPS.16 of HBV S gene fragments were characterized byusing this method.14 of them were positive for nt551A, one was positive for nt551G, and the other one was positive for nt551T.The results were confirmed by nucleotidesequencing CONCLUSION: The mutation specific polymerase chain reaction is a specific and sensitive method for detecting the mutations of HBV genome at nt551.