Boucher JI, Whitfield TW, Dauphin A, Nachum G, Hollins C, Zeldovich KB, Swanstrom R, Schiffer CA, Luban J, Bolon DNA. Constrained Mutational Sampling of Amino Acids in HIV-1 Protease Evolution. Mol Biol Evol. 2019 04 01; 36(4):798-810.