This invention relates to a nucleic acid molecule encoding a middle Hepatitis B virus (HBV) surface protein, a vector comprising the nucleic acid molecule, a host cell comprising the vector, and a composition comprising the expression products of this vector, which may comprise middle HBV surface protein, or a mixture of middle HBV surface protein and small HBV surface protein. The compositions of the invention may be useful for expressing a middle HBV surface protein, or a mixture of small and middle HBV surface proteins in defined ratios, determining the binding of an antibody to a middle or small HBV surface protein, determining the quality of an anti-middle or an anti-small HBV surface protein antibody, or determining the quality of a kit containing anti-middle or anti-small HBV surface protein antibodies.
