The present invention provides polynucleotide constructs, e.g. recombinant viral vectors, for treating a host cell for a hyperproliferative disorder or infection by an infectious agent, said infection or hyperproliferative disorder being characterised by a human disease-associated trans-acting regulatory factor capable of regulating expression of genes. In particular, the present invention provides such constructs comprising:

(i) at least two tandem copies of a cis-acting regulatory sequence which are controllable by said trans-acting regulatory factor; and
(ii) an effector gene under the control of said cis-acting regulatory sequence the expression of which renders said cell susceptible to protection or destruction, said effector gene encoding a gene product which is capable of converting a prodrug into its active drug form.