The present invention provides, inter alia, recombinant chimeric nucleic
acids encoding a Gag-fs-fusion partner fusion protein; a pseudovirion
comprising a retroviral Gag protein and a fusion partner, wherein the
fusion partner is present in a Gag-fs-fusion partner fusion protein; an
immunogenic composition comprising a pseudovirion; a Gag-fs-fusion partner
fusion protein; and a method of making the pseudovirions of the present
invention.