A cis-acting nucleotide sequence which is capable of rendering the removal of introns from a precursor transcript encoded by a gene, which gene harbors at least one such cis-acting nucleotide sequence, occurring during the production of mRNA of a gene, dependent upon activation of a trans-acting factor. The trans-acting factor is an RNA-activated protein kinase which is capable of phosphorylating the -subunit of eukaryotic initiation factor 2. The trans-acting factor is preferably, the RNA-activated protein kinase (PKR). The cis-acting nucleotide sequence can be derived from the 3 untranslated region of the human tumor necrosis factor gene (TNF-3-UTR) and may comprise the nucleotide sequence as denoted by SEQ ID NO:1 or biologically functional fragments, derivatives, mutants and homologues thereof.