Discloses a composition comprising an expression vector comprising a first polynucleotides encoding an optimised Feline Leukemia Virus (FeLV) envelope (ENV) polypeptide and a second polynucleotide encoding an FeLV GAG/PRO polypeptide, wherein the optimised FeLV ENV polypeptide comprises a mutation at amino acid position 527 or equivalent corresponding amino acid position of an FeLV ENV protein, and wherein the mutation comprises a substitution of arginine (R), aspartic acid (D), or methionine (M) for glutamic acid (E), and wherein the polynucleotide encodes an optimised FeLV ENV polypeptide having the sequence as set forth in SEQ ID NO: 2, 4, 27, 28, 29, 30, 31, 32, 33 or 34, wherein the sequences are as defined in the complete specification.
