There is disclosed a peptide mixture suitable for eliciting an immune response. It consists of six peptides each corresponding to a fragment of the RAS protein wherein each peptide comprises a region of at least 8 amino acids which includes position 12 of the RAS protein. Each of said regions of the peptides 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. Each of the peptides has an amino acid substitution at the amino acid corresponding to said position 12, selected from a G12A, G12C, G12D, G12R, G12S or a G12V substitution, and the amino acid substitution of each peptide is different from the amino acid substitution of the other peptides..
