DNA constructs comprise a first exon sequence of nucleotides encoding a first peptide or polypeptide, a second exon sequence of nucleotides encoding a second peptide or polypeptide and a third sequence of nucleotides between the first and second sequences encoding a heterologous intron, for example that of Tetrahymena thermophila nuclear pre-rRNA, between RNA splice sites and a site-specific recombination sequence, such as loxP, within the intron, the exons together encoding a product peptide or polypeptide. Such constructs are of use in methods of production of peptides or polypeptides, transcription leading to splicing out of the intron enabling translation of a single chain product peptide or polypeptide. Isolated nucleic acid constructs consisting essentially of a sequence of nucleotides encoding a self-splicing intron with a site-specific recombination sequence within the intron, for use in creation of constructs for expression of peptides or polypeptides, are also provided.