The protease encoded by the human immunodefiency virus (HIV) is a homodimer as determined by X-ray crystallographic (Navia et al., 1989, Wlodawer et al., 1989) and biochemical analysis (Meek et al., 1989). A four-stranded antiparallel s-sheet generated by interdigitating N-and C-termini of the monomers dominates the dimer interface. This s-sheet is partially stabilized by intersubunit backbone H-bonds of alternate amino acids from each of the four strands.