Fusion proteins comprise a first amino acid sequence and a second amino acid sequence. The first amino acid sequence is derived from a retrotransposon or an RNA retrovirus and confers on the fusion protein the ability to assemble into particles; an example is the product of the TYA gene of the yeast retrotransposon Ty. The second amino acid sequence is biologically active; for example it may be antigenic. So particles formed of the fusion proteins may be useful in vaccines or in diagnostic or purification applications.