There is disclosed a peptide mixture suitable for eliciting an immune response. It comprises a first, a second and at least one further peptide, each corresponding to a fragment of the RAS protein. Each of the first and second peptides comprises a region of at least 8 amino acids which includes position 13 of the RAS protein, and each of said regions independently has at least 6 amino acid residues, other than at said position 13, which are identical to the corresponding region of the RAS protein. Each of the first and the second peptides has an amino acid substitution at the amino acid corresponding to said position 13, wherein the amino acid substitution of the first peptide is different from the amino acid substitution of the second peptide. The at least one further peptide comprises a region of at least 8 amino acids which includes position 12 of the RAS protein, wherein each of said regions independently has at least 6 amino acid residues, other than at said position 12, which are identical to the corresponding region of the RAS protein, and wherein there is an amino acid substitution at the amino acid corresponding to position 12 of the RAS protein; and/or the at least one further peptide comprises a region of a least 8 amino acids which includes position 61 of the RAS protein, wherein each of said regions independently has at least 6 amino acid residues, other than at said position 61, which are identical to the corresponding region of the RAS protein, and wherein there is an amino acid substitution at the amino acid corresponding to position 61 of the RAS protein.