There is disclosed a peptide for use as a vaccine or medicament and which corresponds to a fragment of the RAS protein. The peptide comprises a region of at least 8 amino acids which includes position 13 of the RAS protein, and said region has at least 6 amino acid residues, other than at said position 13, which are identical to the corresponding region of the RAS protein. The peptide has a G13C or a G13R amino acid substitution at the amino acid corresponding to said position 13.
