1) Retinol and its derivatives; C(20) polyunsaturated, cyclic, lipid soluble alcohol; deficiency results in night blindness and certain epithelial disorders.

2) A group of tetraterpenes, with four terpene units joined head-to-tail. Biologically active members of this class are used clinically in the treatment of severe cystic acne; psoriasis; and other disorders of keratinization.

3) Vitamin A or a vitamin A-like compound.