The present invention provides an anti-cancer vaccine comprising: (i) at least one peptide comprising the amino acid sequence of a neoantigen encoded by a mutant homologous recombination (HR) DNA repair gene selected from the group: BRCA1, BRCA2, PALB2, CDK12, RAD51B, RAD51C and RAD51D, wherein the mutant gene comprises a reversion mutation; and/or (ii) at least one polynucleotide encoding the at least one peptide of (i). Also provided are engineered T cells that recognise said neoantigen. Related methods and medical uses of the vaccine and/or engineered T cell are provided, including for the treatment of cancers, such as homologous recombination (HR) deficient cancers that acquire PARP inhibitor resistance or platinum resistance by development of reversion mutations in an HR DNA repair gene selected from the group: BRCA1, BRCA2, PALB2, CDK12, RAD51B, RAD51C and RAD51D.