However, each of these proteins has its own characteristics: Fel d 1 is present in all cat species and has only limited homology with other mammalian proteins; by contrast Der p 1 is a potent cysteine protease and many authors have argued that the enzymatic activity of this protein plays an important role in its ability to initiate the IgE response to dust mite allergens.40, 41, 42 Our data supports the view that the “minor” components of both dust mite and cat extracts can have a greater role in asthma because they induce smaller quantities of sIgG4 antibodies. This evidence concerns the gene CTSB and asthma.