The present invention provides a construct which, when expressed in a host cell, is capable of producing empty virus capsids, the construct comprising: (i) a nucleotide sequence encoding a capsid precursor protein; (ii) a nucleotide sequence encoding a protease capable of cleaving the capsid precursor protein; and (iii) a control element which controls the expression of the protease such that, when the construct is present in the host cell, the control element causes the protease to be expressed at a level sufficient to cleave the capsid precursor protein, but not sufficient to induce significant toxicity in the host cell. The invention also provides a vector and a host cell comprising such a construct and their use to generate empty virus capsids.