A nucleic acid vector comprising: (i) a promoter； (ii) a sequence encoding an HML-2 polypeptide operably linked to said promoter； and (iii) a selectable marker. Preferred vectors comprise: (I) an eukaryotic promoter； (ii) a sequence encoding an HML-2 polypeptide downstream of and operably linked to said promoter, (iii) a prokaryotic selectable marker； (iv) a prokaryotic origin of replication； and (v) an eukaryotic transcription terminator downstream of and operably linked to said sequence encoding an HML-2 polypeptide. Vectors of the invention are particularly useful for expression of HML-2 polypeptides either in vitro (e.g. for later purification). Or in vivo (e.g. for nucleic acid immunization). They are well suited to nucleic acid immunization against prostrate tumors. A preferred HML-2 is PCAV, which is located in chromosome 22 at 20.428 megabases (22q11.2). A virus-like particle (VLP) comprising HML-2 gag polypeptides and its use in immunization and treatment or diagnosis of prostate tumor.
