Under the selection pressure of drugs, mutations appear in HIV-1 protease even at the sites, which are conserved in the untreated individuals. Cysteine 95 is a highly conserved residue and is believed to be involved in regulation of HIV-1 protease. In some of the virus isolates from patients undergoing heavy treatment with anti-HIV protease drugs, C95F Mutation has appeared. The present study reports 1.8 Angstrom X-ray structure of C95M/C1095F double mutant of tethered HIV-1 protease dimer complexed with acetyl pepstatin. It is found that in this Mutant, dimer interface has become more rigid and that the packing at the interface of terminal and core domains is altered. These alterations may be relevant to C95F mutation conferring drug resistance to HIV-1 protease. (C) 2004 Elsevier Inc. All rights reserved.