The mature cysteine protease from Dermatophgoides pteronyssinus, Der p 1, is a major house dust mite allergen. Its enzymatic activity has been shown to have pro-inflammatory effects that could also negatively influence efficacy of allergen-specific immunotherapy. The aim of this study was to express recombinant pro-Der p 1 (rpro-Der p 1) in the yeast Pichia pastoris and to study its maturation.