Positions of mutation in chimeric 12X FIV protease. The residues that were mutated to generate the 12X mutant of FIV protease are indicated in yellow. These included I37V in the active site core, N55M, M56I, I57G, V59I, G62F, and K63I in the flap region, and L97T, I98P, Q99V, P100N, and L101I in the "90s loop" region. The 2-fold axis of the 12X FIV protease dimer is vertical in the plane of the figure; the C2 axis of the bound inhibitor, TL-3, coincides with this 2-fold. All figures were generated using MoViT version 1.2.1 (Pfizer, La Jolla, CA, USA).