Cytochrome P450 is a large family of enzymes that are found in both prokaryotic and eukaryotic organisms. In mammalian cells, they are found in both the ER and the mitochondria, and they function in various biosynthetic pathways, such as cholesterol and steroid hormone synthesis. Several dozen CYP450 (CYP) isoforms are involved in drug metabolism; they are the most important group of enzymes in phase I metabolism. These isoforms are found mostly in the ER.