Conservedphenylalanine 35 is one of the hydrophobic patch residues on the surface of cytochromeb5 (cytb5). This patch is partially exposed on the surface of cyt b5while its buried face is in direct van der Waals’contact with heme b. Resi-dues Phe35 and Phe/Tyr74 also form an aromatic channel with His39, which is one of the axial ligands of heme b. By site-directedmutagenesiswehaveproduced threemutantsof cytb5:Phe35fiTyr, Phe35fiLeu, and Phe35fiHis.