Cat Allergen, Fel D 1 is a secretoglobin protein and the primary allergen of cats (Felis domesticus). It is mainly produced in the saliva and by the sebaceous glands located in the skin. It causes an IgG or IgE reaction in humans which can induce a variety of symptoms from sinusitis to severe asthma. It is the second major indoor allergen after dust mites. All cats produce Fel d 1, but hormonal status impacts its production and unnurtured males produce more Fel d 1 than females. Diagnostics for cat allergy have traditionally been based on cat epithelial extracts which are highly variable depending on their source. IgE reactivity to Fel d 1 accounts for 88% of the total IgE response to cat allergens and yeast expressed offers an ideal alternative to natural cat allergen nFel d.