Abstract: A DNA expression vector which contains the trp promotor is described. The expression vector provides for the overproduction of .beta.-lactamase. Insertion of a gene or cDNA into the .beta.-lactamase gene of the expression vector results in the over-production of a fusion protein comprising a part of the .beta.-lactamase as the N-terminal end and the protein coded for by the inserted DNA as the C-terminal end. Using the expression vector described herein, it is possible to obtain large amounts of the fusion protein. A fusion protein containing the surface antigen of Hepatitis B virus and a vaccine containing this fusion protein are also described.