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Heterologous expression systems have proven to be very important in P450 research, permitting investigation of many P450s identified only by recombinant (cloning) technologies. They also make it possible to study human P450s since tissue sources for naturally occurring enzymes are difficult to obtain. A variety of different heterologous systems have been applied to the study of P450s and each one has its own unique advantages. For the study of biophysical properties and structure/function relationships, E. coli has proven to be a particularly good system. Both microsomal and mitochondrial P450s can be expressed to high levels in E. coli and subsequently purified to homogeneity for detailed analysis. Techniques of both site directed mutagenesis and random chimeragenesis are very facile in bacteria providing excellent opportunity to analyze specific aspects of structure/function relationships of P450s. Furthermore microsomal P450s are active in intact E. coli, activities being supported by flavodoxin and flavodoxin reductase, providing the opportunity to develop bioreactors expressing designer P450s. The salient features of P450 expression in bacteria are summarized herein.