Fel d 1 is a secretoglobin protein that, in cats, is encoded by the CH1 (chain 1/Fel d 1-A) and CH2 (chain 2/Fel d 1-B) genes. Among cats, Fel d 1 is produced largely in their saliva and by the sebaceous glands located in their skin. It is the primary allergen present on cats and kittens. The function of the protein for cats is unknown, but it causes an IgG or IgE reaction in sensitive humans (either as an allergic or asthmatic response).