The way in which structural diversity encodes the capacity of individual P450 enzymes to metabolize multiple, structurally distinct substrates remains largely unknown. The tools of molecular biology provide a means of identifying amino acid residues among closely related P450s that are determinants of their distinct catalytic properties. Work in our laboratory has identified two substrate specificity-determining segments of the amino acid sequences of subfamily 2C P450s. A pattern has emerged from this work, and that of others, which suggests a model for the structural basis of P450 catalytic diversity.