This chapter discusses designer membranes and its construction of a cell containing multiple membrane-bound cytochromes p450. The techniques of molecular biology offer the opportunity to introduce foreign DNA into naive cells to express enzymatically active proteins that can be tested in situ. Indeed, one is able to introduce simultaneously the DNAs for a number of different soluble and membrane-bound proteins, and thereby construct a functional metabolic pathway in a cell not programmed for such activities. Cytochrome P450 is best characterized as the catalyst for many monooxygenase—mixed function oxidase—reactions. For these reactions, a P450 serves as the central agent for the binding of a molecule of substrate, the acceptance of reducing equivalents provided by electrons transferred from reduced pyridine nucleotide via an abbreviated electron transport chain, and the activation of a molecule of molecular oxygen. The use of heterologous expression of enzymes, in particular membrane-bound enzymes, at present offers the possibility to contribute new types of data needed for the better understanding the functioning of enzymes in the environment of a cell.