RESUMEN
<p><b>BACKGROUND</b>To study the difference between the mutant types and the wild type of HBsAg on the binding efficiency with antibodies in order to find some methods to detect the mutants.</p><p><b>METHODS</b>The recombinant wild type of HBsAg and the mutants including 145 R, 133 T, 126 S, 141 E, 126 S+133 T, 126 S+133 T+145 R and 126 S+133 T+141 E were cloned, respectively. Then the expression vector containing the wild or mutant gene was transfected into COS7 cells, and the recombinant proteins were expressed. The proteins were detected with the routine HBsAg kits.</p><p><b>RESULTS</b>The binding efficiency with monoclonal antibodies of HBsAg expressed by 126 S was higher than that of the wild type, while the results of 145 R, 141 E, 126 S+133 T+145 R and 126 S+133 T+141 E were lower than that of the wild, and 133 T was similar to the wild. But most of the mutants had the same reactivity with the polyclonal antibody.</p><p><b>CONCLUSIONS</b>The effect of mutations on antibody binding differs depending on the amino acid involved and on the location within the "a" loop. So it is necessary to use polyclonal antibody or to find new kits to detect the mutants of HBsAg.</p>