“…Taking advantage of the high acid stability of PR, the enzyme was purified in a one-step acid procedure (von der Helm et al, 1994). As reference for the establishment of the test system, recombinant HIV-1 wild-type PR (clade B MVP899-87 isolate; Gürtler et al, 1994) was expressed and purified as above and then further purified in a second step resulting in > 70% purity (von der Helm et al, 1994;Gehringer et al, 2003). Preparation of recombinant PR from patient plasma takes 5 days and can be paused and continued later at several preparation levels of the protocol.…”