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. or A mixture of two or more different kinds of cells that are grown together.