Recombinant VP2 proteins of HBoV1, 2, 3 and 4 were also expressed in E. coli Rosetta (DE3) (Novagen, Madison,WI,) cells and purified as previously described [37] . The genes used in prokaryotic cloning were the same as those used in a baculovirus expression system. The recombinant antigens were used to immunize mice to produce antibodies (see below).