The invention relates to a peptide capable of activating NK cell-mediated immunity, the peptide comprising or consisting of the amino acid sequence XnAX2X1, Wherein Xn is an amino acid sequence of between 5 and 12 residues, and X1 is any amino acid; or leucine or phenylalanine; and X2 is alanine, threonine or serine. The invention further relates to an MHC class I molecule and the peptide, nucleic acids encoding the peptide, activated NK cells, and related compositions and methods, including use in methods of treatment.