A hypothetical mechanism for the one-carbon ring expansion of aziridines is shown in Figure . The reaction of a hemoprotein with a suitable carbene precursor forms an electrophilic iron-carbenoid intermediate, which could be trapped by a sufficiently nucleophilic aziridine. Ammonium ylides are commonly proposed as intermediates in hemoprotein-catalyzed N-H insertion reactions, 22 and Schomaker has reported numerous examples where carbamateprotected aziridines react with metal-carbenoid electrophiles to form aziridinium ylides. At the present time, it is not clear whether this intermediate would exist as a "free" or metal-bound ylide. Finally, the aziridinium ylide could undergo the desired [1,2]-Stevens rearrangement preferentially over cheletropic extrusion of ethylene, liberating the desired product and regenerating the hemoprotein. We envisioned that the active site of an enzyme could mimic solvent caging effects, which are known to exert selectivity over radical recombination in [1,2]-Stevens rearrangements, to achieve asymmetric induction during ring expansion. Hemoproteins demonstrate high stereoselectivity in radical reactions, both in their native reactivity as well as in new-to-nature activity cultivated through protein engineering, lending further support to this hypothesis.