The present invention relates to codon optimised polynucleotides which are efficiently expressed in mammalian cells and encode insect proteins from Dermaphagoids dust mite. In particular, the optimised codon polynucleotides encode a protein from Dermaphagoides pteronyssinus, such as DerP1 or proDerP1. The present invention also provides methods of preparing pharmaceutical compositions comprising the expression of the codon optimised polynucleotides, and vectors and transformed host cells comprising them.