When Fel d 1 was first discovered, the proposed name was “cat allergen 1.” The name Fel d 1 comes from Felis domesticus, which was the commonly used species name of the domestic cat at the time the allergen was officially named.  Although the domestic cat’s official species name is now Felis catus, the Fel d name has not been changed. The Fel d allergens were numbered in order of their discovery; Fel d 1 was the first.