There is disclosed a peptide mixture suitable for eliciting an immune response. It comprises a first and a second 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. Each of said regions of the first and second peptides 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 a point mutation at the amino acid corresponding to said position 13. The point mutation of the first peptide is different from the point mutation of the second peptide.