At least one nucleic acid construct encoding a first conserved element polypeptide, a second nucleic acid construct encoding a second conserved element polypeptide and a nucleic acid construct encoding a full-length, or substantially full-length, protein of interest, for use in a method of inducing an immune response to the protein of interest, wherein the protein of interest is HIV-1 Gag or HIV-1 Env and wherein the substantially full-length protein is a region of the HIV-1 Gag or HIV-1 Env protein that includes all of the conserved elements, and the method comprises: priming an immune response to the protein of interest, wherein priming comprises administering; the at least one nucleic acid construct encoding the first conserved element polypeptide, wherein the conserved elements are from the protein of interest and the polypeptide comprises at least three conserved elements, each from 8 to 50 amino acids in length where the conserved elements are joined by peptide linkers that do not naturally occur in the protein of interest; and the nucleic acid construct encoding the second conserved element polypeptide that comprises at least one variant of a conserved element contained in the first conserved element polypeptide, wherein the variant in the second polypeptide differs from the first polypeptide by 1, 2, 3, 4, or 5 amino acids, with the proviso that a nucleic acid construct encoding the full-length protein, or a substantially full-length region thereof, is not administered in the priming step; and boosting the immune response to the protein of interest, wherein boosting comprises co-administering (i) the nucleic acid construct encoding the full-length protein of interest, or substantially full-length protein of interest, and (ii) the nucleic acid construct encoding the first conserved element polypeptide and the nucleic acid construct encoding the second conserved element polypeptide. priming an immune response to the protein of interest, wherein priming comprises administering; the at least one nucleic acid construct encoding the first conserved element polypeptide, wherein the conserved elements are from the protein of interest and the polypeptide comprises at least three conserved elements, each from 8 to 50 amino acids in length where the conserved elements are joined by peptide linkers that do not naturally occur in the protein of interest; and the nucleic acid construct encoding the second conserved element polypeptide that comprises at least one variant of a conserved element contained in the first conserved element polypeptide, wherein the variant in the second polypeptide differs from the first polypeptide by 1, 2, 3, 4, or 5 amino acids, with the proviso that a nucleic acid construct encoding the full-length protein, or a substantially full-length region thereof, is not administered in the priming step; and boosting the immune response to the protein of interest, wherein boosting comprises co-administering (i) the nucleic acid construct encoding the full-length protein of interest, or substantially full-length protein of interest, and (ii) the nucleic acid construct encoding the first conserved element polypeptide and the nucleic acid construct encoding the second conserved element polypeptide.