The threshold values for allergen exposure most widely accepted are as follows: 2 μg or more of Group 1 mite allergens per gram of dust (Der p 1, Der f 1) as a risk factor for development of IgE antibody and asthma, and a higher level of 10 μg Group 1 mite allergens per gram of dust as a risk factor for an acute attack of asthma; 1 μg or more of cat allergen (Fel d 1) per gram of dust as a risk factor for sensitization to cats, and a higher level of 8 μg cat allergen per gram of dust as a risk factor for asthma symptoms in most cat allergic patients [21,28]. This evidence concerns the gene IGHE and asthma.