Protease
HIV-1 protease is a homodimer of 99 amino-acids: (wikipedia)
Cleavage of polyproteins is essential to HIV lifecycle
Protease Inhibitors bind to the active site: (wikipedia)
Resistance mutations lead to drug resistance: [Molla et al, Nature Medicine 2, 760 (1996)]
Mutations appear reproducibly at sites 82, 54, 71.