Castoreum /kæsˈtɔːriəm/ is a yellowish exudate from the castor sacs of mature beavers. Beavers use castoreum in combination with urine to scent mark their territory.[1][2] Both beaver sexes have a pair of castor sacs and a pair of anal glands, located in two cavities under the skin between the pelvis and the base of the tail.[3] The castor sacs are not true glands (endocrine or exocrine) on a cellular level, hence references to these structures as preputial glands, castor glands, or scent glands are misnomers.[4]