Abstract Cytochromes P450 constitute a superfamily of hemoproteins which have evolved from a common ancestor. Recent advances in molecular biology have offered a powerful approach to the research on the multiplicity of P450. Now, every mammalian P450 species can be characterized by heterologous expression of its cDNA. In addition, a heterogous expression system is also useful for analysis of structure-function relationships of P450 monooxygenases. Comparison of enzymatic activity and expression level in yeast of a series of artificial genetic fusion enzymes of P450 with P450 reductase has revealed the strategy of how to construct a most suitable fusion. The P450/reductase fused enzyme is a simplified monooxygenase as compared with the two enzyme systems in nature. The P450 domain of the fused enzyme is bound to the microsomes, while the reductase domain lies on the cytoplasmic side, moving flexibly. This structural feature seems to reflect the topology of both enzymes in mammalian microsomes, and will be used as a model to analyze in vivo protein-protein interactions of microsomal P450 monooxygenases. Combined with the discovery of some naturally occurring P450/reductase fusions from bacteria to mammals, comparison of these natural enzymes with artificial ones will be discussed.