Including BCG in such heterologous prime-boost regimes allows for the beneficial effects of BCG to be retained. We have developed an immunisation strategy using BCG as the priming immunisation and a recombinant MVA (rMVA) as the boost. The antigen selected for inclusion in the rMVA must be present in all strains of BCG. We have selected Antigen 85A, which forms part of the immunodominant antigen 85 complex. Although there are numerous candidate antigens for use in a TB vaccine, antigen 85A has long been considered a leading choice. It is a major target antigen recognised by T cells from infected individuals and it is protective as a DNA vaccine in small animals. Importantly, for use in BCG prime-MVA boost regimes, it is highly conserved amongst all mycobacterial species and is present in all strains of BCG. In small animals it is a major target of the immune responses induced by BCG and in humans HLA-A2 restricted CD8 T cells have been found in a high proportion of BCG-immunised individuals. Antigen 85A is an enzyme, mycolyl transferase, which is involved in cell wall biosynthesis.