1) A naturally occurring benzoquinone important in electron transport in mitochondrial membranes. Coenzyme-Q functions as an endogenous antioxidant; deficiencies of this enzyme have been observed in patients with many different types of cancer and limited studies have suggested that coenzyme-Q may induce tumor regression in patients with breast cancer. This agent may have immunostimulatory effects.

2) A substance found in most tissues in the body, and in many foods. It can also be made in the laboratory. It is used by the body to produce energy for cells, and as an antioxidant. It is being studied in the treatment of cancer and in the relief of side effects caused by some cancer treatments. Also called coenzyme Q10, Q10, CoQ10, and vitamin Q10.