*The cytochrome P450 isozymes play important roles in metabolism and steroidogenesis.
*The multiple forms of phosphodiesterase also play major roles in various biological processes.  Although more than one form of these enzymes have been found in individual cells, these isoforms of the enzyme are unequally distributed in the various cells of an organism.  From the clinical standpoint they have been found to be selectively activated and inhibited, an observation which has led to their use in therapy.