This invention relates to novel HIV polypeptide and polynucleotide fusions of Gag, Pol and Nef which are useful in immunogenic compositions and vaccines. The invention relates in particular to a polypeptide which comprises Nef or an immunogenic fragment thereof, and p17 Gag and/or p24 Gag or immunogenic fragments thereof, wherein when both p17 and p24 Gag are present there is at least one HIV antigen or immunogenic fragment between them. The polypeptide may also comprise Pol or RT or an immunogenic fragment thereof.