The present invention relates to the tumor-specific expression of heterologous polynucleotides, particularly genes encoding cytotoxic products, under the control of transcriptional regulatory sequences. Particularly preferred transcriptional regulatory sequences comprise H19, IGF-1 and IGF-2 P4 transcriptional regulatory sequences, and variants thereof. The present invention also provides expression constructs and methods of administering such expression constructs. The compositions and methods of the invention are useful for the treatment of proliferative disorders, particularly cancer.