Fel d 1 is a secretoglobin protein complex 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). Variation in cats Kittens produce less Fel d 1 than adult cats. Female cats produce a lower level of Fel d 1 than (unneutered) males, while neutered males produce levels similar to those of females. Both intact and spayed females produce similar levels. Although females and neutered males produce Fel d 1 in lower levels, they still produce enough to cause allergic symptoms in sensitive individuals. Researchers have been investigating reports from cat owners that certain breeds of cats either do not produce Fel d 1 or are thought to do so at significantly lower levels than other breeds. For instance, individual cats from the naturally occurring Siberian breed, native to the Siberian region for which the breed is named, have been shown to have genetic variants that result in a lower production of Fel d 1. Another breed thought to have a possible genetic disposition not to produce this allergen or to produce less of it is the Balinese, an offshoot of the Siamese breed. Several other breeds are widely referenced as causing a diminished immune reaction in cat allergy sufferers, including Sphynx, Russian Blue, Cornish Rex, Devon Rex, Siamese, Javanese, Oriental shorthair, Burmese, and Laperm. Fairly reliable tests for their Fel d 1 protein production is available for individual cats, but research regarding entire breeds continues, hampered by the lack of a thoroughly accessible and accurate genetic test for production of the antigen. Structure The complete quaternary structure of Fel d 1 has been determined. The allergen is a tetrameric glycoprotein consisting of two disulfide-linked heterodimers of chains 1 and 2. Fel d 1 chains 1 and 2 share structural similarity with uteroglobin, a secretoglobin superfamily member; chain 2 is a glycoprotein with N-linked oligosaccharides. Both chains share an all alpha-helical structure. Presence in other species Proteins matching the InterPro family signature for Fel d 1 parts is widespread among Theria, a subclass of