We have aligned the sequences of the three P450s based on the structural superposition. Figure 3 shows this sequence alignment, with the positions of helices, β –strands, and the highly conserved β–bulge or ‘Cys-pocket’ indicated. As in Figure 1 , it is clear that the same set of secondary structures is maintained in all three molecules. It is also evident that both P450terp and P450BM-3 contain several large insertions compared with P450cam . It is noteworthy that, with the exception of the amino terminus, P450cam features no significant insertions relative to either P450terp or P450BM-3 . In a comparison of these three structures with a large number of P450s, P450cam seems to represent the minimal P450 structure. As discussed below, the location of the insertions in P450terp and P450BM-3 is significant in relation to the function of P450s.